COMPARATIVE ANATOMY. 



17 



COMPARATIVE ANATOMY. I. 



INTRODUCTION TERMS EMPLOYED IN CLASSIFICATION. 



; :mple instructions given by Linni to all succeeding natu- 



" Observe and compare." This Swedish natnmli-t. 



: LinwBUS, assiduously followed his own maxim, and 



became one of the greatest masters of the description, and the 



contributor to the science of the classification, of living 



..In. in the world has known. 



10 higher animals are free, locomotive, well-defined indi- 

 viduals. Euoh has within tho circumscribed limits of its body, 

 whether that body be of moderate dimensions or extremely 

 minute, every organ which is requisite to self-existence and 

 reproduction. The actions which the body has to perform in 

 order to carry on that orderly system of constructive change 

 which is always associated with life, are very numerous. To 

 jMTl'urm those actions, many complex organs are required; 

 hence an animal is a very compact piece of machinery, no part 

 of which can be dispensed with without crippling the whole. As 

 in a large factory every band, and wheel, and rod, from the 

 great piston to tho little bobbin, has its separate office, the 

 adaptations to which have required thought and contrivance ; 

 o there is no part of any animal which is not fitted to carry out 

 uomo necessary function. 



The outward form of animals is often beautiful, and the study 

 of it instructive; but it is obvious that we cannot expect to 

 know anything of the animal, considered as a machine, until we 

 have searched it throughout by cutting down to every internal 

 organ, and examining all tho peculiarities of each. If we neglect 

 to do this, it is not only probable, but certain, that in the un- 

 xamined part we shall leave some secret of its life, some 

 admirable contrivance, some wonderful adaptation, unnoticed. 

 This leads us to the conclusion that in order to acquire a know- 

 ledge of living things we must use the knife. The microscope, 

 the injecting syringe, and all the appliances of modern science, 

 may be used, but the knife or scalpel is indispensable, and the 

 use of it has given a name to the science. The word anatomy 

 is derived from the Greek ava (an'-a), through, and rofji-rj (tom'-e), 

 a cutting. In following the Linnaean direction to observe in 

 this realm of Nature, it was natural that the only means of ob- 

 servation should give its name to the science which sprung out 

 of the investigation. At first, however, tho study was directed 

 upon one species only. If in more senses than one the proper 

 tudy of mankind is man, it was natural that at first the human 

 frame should have monopolised all the attention of scientific 

 dissectors. Hence the word anatomy was applied to the study 

 of the structure of the human species. As science advanced, 

 other animals were examined in the same way, and tho new 

 study, as it always suggested a comparison with the results of 

 the old, was called comparative anatomy. 



Comparative anatomy is a study of all the parts of all the 

 different kinds of bodies which are found in the animal kingdom, 

 eo far as structure is concerned. Strictly speaking, it treats of 

 the dead animal alone. It describes the machine when the 

 motive power has ceased to act. Nevertheless, in examining 

 the structure of a species it is quite impossible, and very unde- 

 sirable, to exclude the idea of tho function which the several 

 parts have to perform when animated with life. Thus the twin 

 studies of anatomy (or the structure of living beings) and of 

 physiology are indissolnbly connected, though distinct from one 

 another. The mechanist has to do with the several parts of the 

 ngine while they are at rest, but every fitting is constructed 

 with reference to motion. He cannot exclude the idea of motion 

 while he is constructing his machine. He asks himself at every 

 stage, Will it go ? will it do its work well ? The works of God 

 cannot be constructed by man, and their simplest contrivances 

 can scarcely be imitated ; but man can examine and analyse 

 them, and as he does so he will be continually asking himself, 

 How does this structure act in the living animal ? and exclaim, 

 as knowledge dawns upon him, How admirably is this organ 

 constructed to do its work ! 



Tho words comparative anatomy, however, suggest another 

 truth they suggest that living beings may be compared with 

 one another. Every animal might be made a study by itself, as 

 man has been. The fact that man's frame has been the subject 

 of thousands of books, and the object of millions of investiga- 

 tions, and still affords unsolved problems, shows that the study 

 of each species is almost unlimited. On comparing the bodies 



28 N'.E. 



of different animals, it IB found that they are not toUll/ di- 

 imilar structure*. The fint thing which strike* the student is 

 that a very largo number of animals are constructed upon the 

 same ground-plan they differ only in the detail* of their 

 structure. Now, the details of structure are often most appa- 

 rent on tho exterior, while the essential plan lies deeper. The 

 anatomist (i.e., dissector) will often reveal a similarity between 

 two animals which tho zoologist would not suspect. . If we take 

 two animals so utterly dissimilar in size, outward form, and 

 habits as the bat and the pig, and dissect them, we shall find 

 that in the main they are alike. Not only is there a bony axis 

 composed of many joints in the interior of the body of each, 

 which supports the animal, gives origin to the muscles, and 

 protects the nervous matter, but with few and slight exceptions 

 we find bone for bone, muscle for muscle, nerve for nerve, in 

 comparing each point of the internal structure of the two ani- 

 mals. Not only is the fore-limb of a dog built upon the same 

 plan as the arm of a man, but it is essentially more like it than 

 it is to the hind-limb of the same animal. 



The similarity of structure which is found throughout a very 

 large number of animals is the first fact which strikes every 

 candid student of comparative anatomy. It is fortunate for the 

 study that this is the case. If every animal were built np on an 

 independent plan, no one could hope to gain a comprehensive 

 view of the structure of the animal kingdom ; nor would the 

 study be so interesting, for the human mind delights in simili- 

 tudes and generalisations ; moreover, on this likeness of structure 

 all classification of animals depends. 



In pursuing his study, the comparative anatomist finds that 

 while a very large number of animals are constructed after the 

 same pattern, this pattern does not run through the structure of 

 all animals. He finds another multitude of animals which are 

 built upon a plan common to them all, but this plan is quite 

 different from that which characterises the first group. When 

 he has determined the number of these large groups, he finds 

 further that each species in one of these groups is not in the 

 same degree like or unlike every other of the same group. If 

 a, b, c, etc., represent a number of animals in a large group, he 

 finds that o is not as like to a as 6 is to a, so that he can 

 arrange them in something like order, placing one next to that 

 to which it is most like, so as to show that though 2 be to a 

 great extent unlike a, yet it is connected with it by the inter- 

 mediate links. Our student also will find that each species is 

 not in the same degree like or unlike even its next door neigh- 

 bour, as every other two next door neighbours are. In other 

 words, there are gaps in the series, and very useful these gaps 

 are, because they enable us to split up the tens of thousands of 

 species which belong to each group into natural sections. The 

 great groups themselves are probably only caused by very wide 

 gaps ; and these groups are subdivided by less marked gaps 

 into smaller groups, and so on. The reader must always remem- 

 ber that the vast scheme of animated nature is far more complex 

 than any of these poor illustrations express, or else he will be 

 misled by that which was intended to explain it. Perhaps tho 

 best illustration of the relations of animals to one another is that 

 of the richly-branched head of a large tree. In summer, when 

 the leafy covering presents an even surface to the eye, the con- 

 nection of the ultimate twigs is not apparent ; but in winter we 

 can see that a number of twigs spring from one little bough, a 

 number of these boughs spring from a branch, and a number of 

 these branches may bo traced down to where they diverge from 

 the giant fork. 



It follows from this arrangement that a great many things 

 may be said about tho structure of each animal in one group 

 which will be true of all in that group. A great many more 

 facts may be stated of the animals of a smaller group, and so 

 on. Now, these statements are the results of comparative 

 anatomy, and the only true grounds of classification. 



The comparative anatomist has a most difficult task before 

 him, and tho collected wisdom of all comparative anatomists has 

 not saved them from many blunders ; but every student of the 

 science has this satisfaction : he knows that the classification 

 which is being worked out ia not an imaginary but a real one 

 The classification which unites animals into groups within 

 groups, grounded on their likeness more or less to one another, 

 indicates a real and natural relationship in those which are 

 placed together. Whether this classification indicates a mate- 

 rial blood-relationship, or reveals the plan of the Almighty 



