476 



NATURE 



[September ii, 1890 



they can be shown to possess certain organs in a less highly 

 developed condition than their ancestors, or even in a rudi- 

 mentary state. 



Thus a crab as compared with a lobster is degenerate in the 

 matter of its tail, a horse as compared with Hipparion in regard 

 to its outer toes ; but it is neither customary nor ad visible to 

 speak of a crab as a degenerate animal compared to a lobster ; 

 to do so would be misleading. An animal should only bespoken 

 of as degenerate when the retrograde development is well marked, 

 and has affected not one or two organs only, but the totality of 

 its organization. 



It is impossible to draw a sharp line in such cases, and to limit 

 precisely the use of the term degeneration. It must be borne in 

 mind that no animal is at the top of the tree in all respects. 

 Man himself is primitive as regards the number of his toes, and 

 degenerate in respect to his ear muscles ; and between two 

 animals even of the same group it may be impossible to decide 

 which of the two is to be called the higher and which the lower 

 form. 



Thus, to compare an oyster with a mussel. The oyster is more 

 primitive than the mussel as regards the position of the ventricle 

 of the heart and its relations to the alimentary canal ; but ismore 

 modified in having but a single adductor muscle ; and almost 

 certainly degenerate in being devoid of a foot. 



Care must also be taken to avoid speaking of an animal as 

 degenerate in regard to a particular organ merely because that 

 organ is less fully developed than in allied animals. An organ 

 is not degenerate unless its present possessor has it in a less 

 perfect condition than its ancestors had. 



A man is not degenerate in the matter of the length of his 

 neck as compared with a giraffe, nor as compared with an 

 elephant in respect of the size of his front teeth, for neither 

 elephant nof giraffe enters into the pedigree of man. A man is, 

 however, degenerate, whoever his ancestors may have been, in 

 regard to his ear muscles ; for he possesses these in a rudimentary 

 and functionless condition, which can only be explained by 

 descent from some better equipped progenitor. 



Closely connected with the question of degeneration is that of 

 the size of animals, and its bearing on their structure and de- 

 velopment ; a problem noticed by many writers, but which has 

 perhaps not yet received the attention it merits. 



If we are right in interpreting the eggs of Metazoa as re- 

 presenting the unicellular or protozoan stage in their an- 

 cestry, then the small size of the egg may be viewed as re- 

 capitulatory. 



But the gradual increase in size of the embryo, and its growth 

 up to the adult condition, can only be regarded as representing 

 in a most general way, if at all, the actual or even the relative 

 sizes of the intermediate ancestral stages of the pedigree. 



It is quite true that animals belonging to the lower groups are, 

 as a general rule, of smaller size than those of higher grade ; and 

 als-o that the giants are met with among the highest members of 

 each division. Cephalopoda are the highest molluscs, and the 

 largest cephalopods greatly exceed in size any other members of 

 the group ; decapods are at once the highest and the largest 

 crustaceans ; and whales, the hugest animals that exist, or, so 

 far as we know, that ever have existed, belong to the highest 

 group of all, the mammalia. It would be easy to quote ex- 

 ceptions, but the general rule obtains admittedly. 



However, although there may be, and probably is, a general 

 parallelism between the increase in size fiom tihe egg to the 

 adult, and the historical increase in size during the passage from 

 lower to higher forms ; yet no one could maintain that ttie sizes 

 of embryos represent at all correctly those of the ancestors ; that, 

 for instance, the earliest birds were animals the size of a chick 

 embryo at a time when avian characters first declared themselves, 

 or that the ancestral series in all cases presented a steady pro- 

 gression in respect of actual magnitude. 



In the lower animals, e.g., in Orbitolites, the actual size of the 

 several ancestral stages is probably correctly recapitulated during 

 the growth of the adult ; and it is very possible that it is so also 

 in such forms as the solitary sponges. In higher animals, except 

 in the early stages of those forms which are practically devoid 

 of food yolk, and which hatch as pelagic larvae, this certainly 

 does not obtain. 



This is clear enough, but is worth pointing out, for if, as most 

 certainly is the case, the embryos of animals are actually smaller 

 than the ancestral forms they represent, it is possible that the 

 smallness of the embryo may have had some influence on its 



NO. 1089, VOL. 42] 



organization, and be responsible for some of the modifications in 

 the ancestral history ; and more especially for the disappearance 

 of ancestral organs in free-swimming larvae. 



In adult animals the relation between size and structure has 

 been very clearly pointed out by Herbert Spencer. Increased 

 size involves by itself increased complexity of structure ; the 

 determining consideration being that while the surface area of 

 the body increases as the squares of the linear dimensions, the 

 mass of the body increases as their cubes. 



If, for example, we imagine two animals of similar shape and 

 proportions, but of different size ; for the sake of simplicity, we 

 may suppose them to be spherical, and that the diameter of one 

 is twice that of the other ; then the larger one will have four 

 times the extent of surface of the smaller, but eight times its 

 mass or bulk : and it is quite possible that while the extent of 

 surface, or skin, in the smaller animal might suffice for the 

 necessary respiratory and excretory interchanges, it would be 

 altogether insufficient in the larger animal, in which increased 

 extent of surface must be provided by foldings of the skin, as in 

 the form of gills. 



To take an actual instance ; Limapontia is a minute nudi- 

 branchiate, or sea-slug, about the sixth of an inch in length ; it 

 has a smooth body, totally devoid of respiratory processes, while 

 forms allied to it, but of larger size, have their extent of surface 

 increased by branching processes, which often take the form of 

 specialized gills. 



This is a peculiarly instructive case, because Limapontia in its 

 early developmental stages possesses a large spirally-coiled shell, 

 and shows other evidence of descent from forms with specialized 

 breathing organs. We are certainly right in associating the 

 absence of respiratory organs in the adult with the small size of 

 the animal ; and comparison with allied forms suggests very 

 strongly that there has been in its pedigree an actual reduction 

 of size, which has led to the degeneration of the respiratory 

 organs. 



This is an important conclusion : it is a well-known fact that 

 the smaller members of a group are, as a rule, more simply 

 organized than the larger members, especially with regard to 

 their respiratory and circulatory systems ; but if we are right in 

 concluding that reduction in size may be an actual cause of 

 simplification or degeneration in structure, then we must be on 

 our guard against assuming hastily that these smaller and simpler 

 animals are necessarily primitive in regard to the groups to 

 which they belong. It is possible, for instance, that the simpli- 

 fication or even absence of respiratory organs seen in Pauropus, 

 in the Thysanura, and in other small Tracheata, may be a 

 secondary character, acquired through reduction of size. 



An interesting illustration of the law discussed above is 

 afforded by the brains of mammals ; it has been noticed by 

 many anatomists that the extent of convolution, or folding of the 

 surface of the cerebral hemispheres in mammals, is related not 

 to the degree of intelligence of the animal, but to its actual 

 size, a beaver having an almost smooth brain and a cow a highly 

 complicated one. Jelgersma, and, independently of him, Prof. 

 Fitzgerald,^ have explained this as due to the necessity of pre- 

 serving the due proportion between the outer layer of grey 

 matter or cortex, which is approximately uniform in thickness, 

 and the central mass of white matter. But for the foldings of 

 the surface the proportion of white matter to grey matter would 

 be far higher in a large than in a small brain. 



It must not be forgotten, on the other hand, that many 

 zoologists hold the view, in favour of which the evidence is 

 steadily increasing, that the primitive or ancestral members of 

 each group were of small size. Thus Fiirbringer remarks, with 

 regard to birds, that on the whole small birds show more 

 primitive and simpler conditions of structure than the larger 

 members of the same group. He expresses the opinion that the 

 first birds were probably smaller than Archosopteryx, and notes 

 that reptiles and mammals also show in their earlier and smaller 

 types more primitive features than do their larger descendants. 

 Finally, Fiirbringer concludes that "it is therefore the study of 

 the smaller members within given groups of animals which 

 promises the best results as to their phylogeny." 



Again, one of the most striking points with regard to the 

 pedigree of the horse, as agreed on by palaeontologists, is the 

 progressive reduction in size which we meet with as we pass 

 backwards in time from stage to stage. The Pliocene Hipparion 

 was smaller than the existing horse, in fact about the size of a 



■' Cf. Nature, June 5, i8go, p. 125. 



