Holes in the Head
A friend said to me once that he would love to see my filing system. So I took him to my office and said, "This file is for correspondence. Here I keep old manuscripts. Here I have manuscripts in preparation. This is the card file of my books‑of my shorter fiction‑of my shorter non‑fiction‑"
"No, no, no," he said. "That's all trivial. Where do you keep your reference files?"
"What reference files?" said 1, blankly. (I very often say things blankly. I think it's part of my charm‑or maybe naively.)
"The cards on which you list items you may need for future articles or books and then file them according to various subjects."
"I don't do that," I said, growing anxious. "Am I supposed to?"
"But how do you keep things straight in your head, then?"
I was glad to be able to answer that one definitely. "I don't know," I said, and he seemed pretty annoyed with me.
Well, I don't. All I know is that I've been a classifier ever since I can remember. Everything falls into categories with me. Everything is divided and counted up and put into neat stacks in my mind. I don't worry about it; it happens by itself.
Of course, I sometimes worry about the details. For instance, what with one thing or another, the actual number of books I have published has become an issue. I am forever being asked, "How many books have you published?"
But what's a book?
Recently, the second edition of my book The Universe was published. Do I count that as a new book? Of course not. It's updated but the updating doesn't represent enough in the way of change to make me consider the book "new." On the other hand, later, the third edition of my book The Intelligent Man's Guide To Science is being published. I counted the second edition as a new book and I intend to count the third edition as such, because in each case the changes introduced were substantial and as time‑ and energy‑consuming as a new book would have been.
You might think all this is something I can chop and change to please myself, but not exactly. In my book Opus 100, 1 listed my first hundred books in chronological order and that list became "official." But is it correct? Was I right in omitting this or that item from the list or, for that matter, including this item or that.
* The answer is 117 at the moment of writing, if you are dying of curiosity.
Unimportant? Sure, but it does help me sympathize with those classifiers who involve themselves with more intricate matters than a listing of books. For instance --
How do you tell a mammal from a reptile?
The easiest and quickest way is to decide that a mammal is covered with hair and a reptile is covered with scales. Of course, you have to be liberal in making this distinction. Some organisms we consider mammals don't have very much hair. Human beings don't‑but we have some hair. Elephants have even less but they have some. Whales have still less, but even they have some. Dolphins usually have anywhere from two to eight hairs near the tip of the ;nout. Even in those whales where hair is altogether absent, it is present at some time in the fetal development.
And one hair is, in this respect, as good as a million, for any hair at all is the hallmark of the mammal. No creature that we consider to be definitely a non‑mammal has even one true hair. They may have structures that look like hair, but the resemblance disappears if we consider its microscopic structure, its chemical makeup, its anatomical origin, or all three.
A somewhat less useful distinction is that mammals (well, most) bring forth their young alive, while reptiles (well, most) don't. Some reptiles, such as sea snakes, bring forth living young, but in doing so they merely retain the eggs within their body till they hatch. The developing embryos find their food within the egg and the fact that the egg is within the body is a point for security, but not for nourishment.
Mammals, on the other hand, or most of them, feed the developing young out of the maternal bloodstream by means of an organ called the "placenta," in which the mother's blood vessels and the blood vessels of the embryo come close enough to allow molecules to seep across: food from mother to embryo, wastes from embryo to mother. (There is no actual joining of bloodstreams, however.)
A minority of mammals bring forth living, but very poorly developed, young, and these must then continue development in a special maternal, pouch outside the body. A still smaller minority of mammals lay eggs. But even the egg‑layers have hair.
Another point is that mammals feed their newborn young on milk secreted by special maternal glands. This is true even of the non‑placental mammals; even of the egg‑layers. And this is not true for any animal without hair (not one!). Milk seems to be a purely mammalian product and it is this, more than anything else, which seems to have impressed the classifiers. The very word "mammal" is from the Latin mamma, meaning "breast."
Then, too, mammals maintain a constant internal temperature even though the environmental temperature may vary widely. Reptiles, on the other hand, have an internal temperature that tends, more or less, to match that of the environment. Since the internal temperature of mammals is close to 100' F. and is therefore generally higher than the environmental temperature, mammals feel warm to the touch while reptiles feel, by comparison, cold. That is why we speak of mammals as "warm‑blooded" and reptiles as "cold‑blooded," missing the essential point that the internal temperature is constant in the former case and inconstant in the latter.
(To be sure, birds are warm‑blooded, too, but there is no danger in confusing a bird and a mammal. All birds, with out exception, have feathers and all non‑birds, without exception, do not have feathers. ‑And except for birds and mammals, all organisms are cold‑blooded.)
I have by no means listed all the differences between mammals and reptiles, only those that the non‑biologist can tell by looking at the creatures from a distance. If we want to indulge in dissection, we can discover others. For instance, mammals have a flat muscle called the "diaphragm" which divides the chesf from the abdomen. The diaphragm as it contracts, increases the volume of the chest cavity (at the expense of the abdominal cavity, which doesn't care) and helps draw air into the lungs. Reptiles do not have a diaphragm. In fact, no non‑hairy organism does.
So far, so good. But now we pass on to extinct creatures which biologists can study only in fossil form. Paleontologists (those biologists specializing in extinct species) have no hesitation in looking at a fossil and saying that it is reptilian or that it is mammalian. The question at once arises: How?
All the really obvious distinctions can't be used since, in general, all that the fossils offer us are the remains of what used to be bones and teeth. You can't look at a handful of bones and teeth and find traces of hair or breasts or milk or placentae or diaphragms.
All you can do is compare the bones and teeth with those of modern reptiles and mammals and see if there are strictly hard‑tissue distinctions. Then, you might assume that if an extinct creature had bones characteristic of mammals, it must also have had hair, breasts, a diaphragm, and the rest.
Consider the skull. In the most primitive and earliest reptiles, the skull behind the eye was solid bone and on the other side of the bone were the jaw muscles. There was a tendency, however, to expose the jaw muscles and give them freer play, so that many reptiles had openings in the skull bounded by bony arches. The loss in sheer defensive strength was more than made up for by the improvement in the offense represented by larger, stronger jaws that could go snap! more firmly. On the balance, then, the reptiles which happened to develop these openings passed on to greater things.
(Yet evolutionary "advances" are never universal, and never the only answer. One group of reptiles that had no use for a hole in the head, managed to survive for hundreds of millions of years and flourish, after a fashion, even today, though many, many hole‑in‑the‑bead groups have vanished. I'm talking about the turtles, whose jaw muscles are hidden under a solid wall of bone.)
The reptiles developed openings in their skulls in a variety of patterns and, indeed, are classified into groups according to those patterns. This is not because this pattern is of overwhelming physiological importance in itself, but only because it is convenient, since if you have any part at all of a reptile, however long dead,, you are likely to have its skull.
But what about mammals, which are descended from reptiles? They‑have a single opening on either side of the skull just behind the eye bounded on the bottom by a narrow bony arch called the "zygomatic arch."
So the paleontologist can look at a skull and from the nature of the openings tell at once whether it is reptilian or mammalian.
Then, again, the lower jaw of a reptile is made up of seven different bones, fused tightly into a strong structure. The lower jaw of a mammal is a single bone. (Some of the missing bones developed into the tiny bones of the middle ear. This is not as strange as it sounds. If you put your finger at the point where lower jaw meets upper jaw and where the old reptilian bones existed, you will find you are not very far from your ear.)
As for the teeth, those of reptiles tended to be undifferentiated and all alike, of conelike structure. In mammals, the teeth are highly differentiated, cutting incisors in front, grinding molars in back, with tearing canines and premolars between.
Since mammals evolved from reptilian forebears is there any way of recognizing which group of reptiles possessed the distinction of being our ancestors? Certainly no living group of reptiles seems to be descended from anything mammalian or even approaching the mammalian. We must look for some group that left no reptilian descendants at all.
One such group, now entirely extinct (as reptiles), is called the "Synapsida." These had a single skull opening on either side of the head and included members who showed the clear beginnings of mammalism.
There were two important groups of synapsids. The earlier, dating back some three hundred million years, were members of the order "Pelycosauria." The pelycosaurs are interesting chiefly because their skulls seem to show the beginning of a zygomatic arch. Furthermore, their teeth show some differentiation. The front teeth are incisorlike and behind them are teeth that are rather like canines. There are no molars however. The rear teeth are reptilian cones.
After flourishing for fifty million years or so, the pelycosaurs gave way to a group of synapsids of the order "Therapsida." Undoubtedly, the therapsids were descended from a particular species of pelycosaurs.
The therapsids are clearly further on the road to mainmalism than any of the pelycosaurs were. The zygoniatic arch is much more mammal‑like among them than among the pelycosaurs; so much so, in fact, that the feature gives them their name. "Therapsida" is from Greek words meaning "beast opening." In other words, the opening in the skull is beastlike where "beast" is the common term for what zoologists would call mammals.
Further, the teeth are much more differentiated among the therapsids than among the pelycosaurs. A well‑known therapsid which lived about 220 million years ago in South Africa had a skull and teeth that were so doglike that it is called "Cynognathus" ("dog jaw"). The back teeth of Cynognathus are even beginning to look like molars.
What's more, while the chin of the therapsids was made up of seven bones, in typical reptilian fashion, the center bone or "dentary," was by far the largest. The other six bones, three on each side, were crowded toward the joint of the lower jaw with the upper‑on their way to the ear, so to speak.
In another respect, too, the therapsids showed a "progressive" feature. (We tend to call "progressive" anything that seems to move in the direction of ourselves.) Early reptiles, including the pelycosaurs, tended to have their legs splayed out so that the upper part, above the knee, was horizontal. This is a rather inefficient way of suspending the weight of the body.
Not so the therapsids. In their case, the legs were drawn beneath the body, with the upper parts, as well as the lower, tending to be vertical. This makes for better support, allows faster movement with less energy expenditure, and is a typically mammalian characteristic. Apparently, the superior efficiency of the vertical leg meant there was no virtue in particularly long toes. Primitive reptiles tended to have four or even five joints in their middle toes. The therapsids, however, had two joints in the first toes, and three joints in the others. Again, this is the way it is in mammals.
The therapsids, however, did not endure. While we may root for them as our great‑ever‑so‑great‑ grandfathers, the fact is that about two hundred million years ago, the "archosaurs," the creatures representing what we loosely call dinosaurs were coming into their own. As they (no ancestors of ours) rapidly grew in size and specialization, they crowded out the therapsids.
By 150 million years ago, the therapsids were clean gone forever, every single one of them extinct.
Well, not really! Some small therapsids remained, but they had grown so mammal‑like, as nearly as we can tell from the very few fossil remnants left behind, that we don't call them therapsids any, more. We call them mammals.
After the mammals came on the scene, they managed to survive through a hundred million years or so of archosaurian dominance. Then, after the archosaurs vanished, about seventy million years ago, the mammals continued to survive and burst into a flood of differentiation and specialization that made this latest period of Earth's existence the "age of the mammals."
The question now is: Why did the mammals survive when the therapsids generally did not? The archosaurs proved utterly superior to the therapsids; why not to those therapsidian offshots, the mammals, as well? It couldn't have been that the mammals were particularly brainy, because primitive mammals aren't. They are not very brainy even today, much less a hundred million years ago.
Nor could Jt be because of their advanced reproductive system, the bearing of live young, for instance. The development of a placenta, or even of a pouch, did not take place till near the end of the archosaurian dominance. For nearly a hundred million years the mammals survived as egglayers.
It couldn't have been their advanced teeth or legs or anything skeletal that the therapsids had, generally, for none of that helped the therapsids, generally.
Actually, the best guess is that the trick of survival was warm‑bloodedness, the development of a constant internal temperature. The control of the internal temperature meant that a mammal could withstand the direct rays of a hot sun much more easily than a reptile could. It meant that a mammal was warm and agile on a cold morning when reptiles were cold, stiff, and sluggish.
If a mammal carefully confined his activity to the chilly hours or if he were trapped by a reptile in the heat and could escape by darting into the hot sun‑it would tend to survive. But for mammals to have survived in this fashion, their warm‑bloodedness must have been well developed from the start and that couldn't happen overnight.
We might conclude, then, that in addition to those changes in the therapsids that we can see in the skeleton, there must have been additional changes that made warmbloodedness, possible. The mammals survived because of all the therapsids, warm‑bloodedness had developed most efficiently among them.
Are there any signs of the beginnings of such changes among the reptilian precursors of the mammals? Well, a number of species of pelycosaurs had long bony processes to their vertebrae that thrust high into the air. Apparently, skin stretched across these processes so that pelycosaurs possessed a high, ribbed "sail."
Why? The American zoologist Alfred Sherwood Romer has suggested it was an air‑conditioning device (like the huge fanlike ears of the African elephant). Heat is gained or lost through the surface of the body and the pelycosaurian sail can easily double the surface area available. On a cool morning, the sail will pick up the Sun's heat and warm the creature much more quickly than would be the case for a similar organism without a sail. Again, on a hot day, a pelycosaur could stay in the shade and lose heat rapidly through the blood vessels engorging the sail.
The sail, in short, served to make the pelycosaur's internal temperature more nearly constant than was the case in other similar reptiles. Their therapsid descendants had no sails, however, and it couldn't be that they had abandoned temperature control, since their descendants, the mammals, had it in such superlative degree.
It must be that the therapsids had developed something better than the sail. A high metabolic rate to produce heat in greater quantity might be developed and then hair (which is only modified scales) to serve as an insulating device that would cut down heat loss on cold days. They might also develop sweat glands to get rid of heat on hot days in more efficient manner than by means of a sail.
In short, could the therapsids have been hairy and sweaty, as mammals are? We can never tell from the fossils.
And did those species which best developed hairiness and sweatiness become what we call mammals and did they survive where the less advanced other therapsids did not?
Let's look in another direction. In reptiles, the nostrils open into the mouth just behind the teeth. This means that reptiles can breathe with their mouths closed‑and empty. When the mouth is full, breathing stops. In the case of the cold‑blooded reptiles, not much harm is done. The reptilian need for oxygen is relatively low and if the supply is cut off temporarily during eating, so what?
Mammals, however, have to maintain a high metabolic rate at all times if they are to be warm‑blooded, and that means that the oxidation of foodstuffs (from which heat is obtained) must continue steadily. The oxygen supply must not be cut off for more than a couple of minutes at any time. This is made possible by the fact that mammals have a palate, a roof to the mouth. When they breathe, air is led above the mouth to the throat. It is only when they are actually in the act of swallowing that the breath is cut off and 1his is a matter of a couple of seconds only.
It is interesting, then, that a number of late therapsid species had developed a palate. This might be taken as a pretty good indication that they were warm‑blooded.
It would seem then that if we could see therapsids in their living state and not as a handful of stony bones, we would see hairy, sweaty creatures that we might easily mistake for mammals. We might then wonder which hairy, sweaty creatures were reptiles and which were mammals. How would we draw the line?
Nowadays, it might seem, the problem is not a crucial one. All the hairy warm‑blooded creatures in existence are called mammals.‑And yet, are we justified in doing so?
In the case of the placentals and the marsupials, we are surely justified. They developed their placentas and their pouches about eighty million years ago, after the mammals had already existed for some hundred million years. The early mammals must have been egg‑layers and so, therefore, must have been their therapsid forebears. If we want to look for the boundary line between therapsids and mammals, we must therefore look among the hairy egg‑layers.
As it happens, there are still six species of such hairy egg‑layers alive today, existing only in Australia, Tasmania, and New Guinea, islands that split off from Asia before the more efficient placental mammals developed, so that the egg‑layers were spared what would otherwise have been a fatal. competition. The egg‑layers were first discovered in 1792 and for a while biologists found it hard to believe they could really exist. It took a long time before they got over suspecting a hoax‑hairy creatures that laid eggs seemed a contradition in terms.
The best known of the egg‑layers is the "duckbill platypus" (the last part of the name means "flat‑foot" and the first part refers to the horny sheath on its nose that looks like a duck's bill). It is also called "Ornithorhynchus" from Greek words meaning "bird beak." 179
180 These egg‑layers have hair, of course, perfectly good hair, but so (very likely) had at least some therapsids. The egg‑layers also produce milk, although their mammary glands have no nipples and the young must lick the hair where the milk oozes out. However, some therapsid species might also have produced milk in that fashion. We can't tell from the bones.
In some respects, the egg‑layers lean strongly toward the side of the reptiles. Their body temperature is much less perfectly controlled than that of other mammals and some of them possess venom. The platypus, for instance, has a horny spur at each ankle which secretes venom; and though a number of reptiles are venomous, no mammals (other than the egg‑layers) are.
	Then, too, because they are egg‑layers ', they have a single abdominal opening, a "cloaca," which serves as a common passageway for urine, feces, eggs, and sperm. All living birds and reptiles (also egg‑layers) possess cloacae, but no mammals, other than those few egg‑layers, do. For this
reason, the egg‑layers are called "monotremes" ("one hole").
To most zoologists, the hair and the milk spell mammal unmistakably, but the eggs, the cloaca, and the venom are sufficiently reptilian so that the egg‑layers are placed in a subclass "Prototheria" ("first beasts") while all other mammals, marsupials and placentals alike, are in the subclass "Theria" ("beasts").
The question arises, though: Are the monotremes really the first of the mammals, or are they rather the last of the therapsids? Are they really reptiles that have the outer appearance of mammals, as did, perhaps, a number of late therapsid species; or are they mammals that have retained some reptilian characteristics?
This may sound like a purely semantic matter, but zoologists must make decisions in such matters and, if possible, come to agreement over it.
An American zoologist, Giles T. Maclntyre, has recently entered the fray, using skeletal characteristics as the criterion. (We have only the skeleton as direct e~,idence in the therapsid case.) He has concentrated on the region near the ear, where some of the reptilian jawbones became mammalian ear bones and where you might expect some useful distinction between the two classes.
There is a "trigeminal nerve" which leads from the jaw muscles to the brain. In all reptiles, without exception, it passes through a little hole in the skull that lies between two particular bones that make up the skull. In all marsupial and placental mammals, without exception, it passes through a little hole that pierces through one of the skull bones.
Then let us forget about hair and milk and eggs and warm‑bloodedness, and reduce it to a matter of holes in the head. Does the trigeminal nerve of the monotremes pass through a skull bone or between two skull bones? The answer has been: Through a skull bone.
That would mean the monotremes are mammals.
Not so, says MacIntyre. The study of the trigeminal nerve was made in adult monotremes, where the skull bones are fused and the boundaries hard to make out. In young monotremes, the skull bones are not as well developed and are more clearly separated (as is true of young mammals generally). In young monotremes, Maclntyre says, it is clear that the trigerninal nerve goes between two bones and it is only in the adult skull that bone fusions obscure the fact.
If MacIntyre is correct, we may therefore say that the therapsids never became entirely extinct and that the monotremes represent living therapsids, living reptiles so similar to mammals in some ways as to have been considered mammals for nearly two centuries.
Does this matter to anyone but a few zoologists?
Well, it matters to me. Emotionally, I'm all the way on MacIntyre's side. I want the therapsids to have survived!