332 COMPARATIVE ANATOMY 



ing adult structure. And while the indispensability of comparative 

 anatomy is clear, it is well to note again how causal morphology 

 would join hands with comparative anatomy here. To recur to the 

 example of the supposed origin of the cbpulatory organs from gills: 

 Due consideration for the functional demands of these two sorts 

 of organs would lead to recognition of the extreme unlikelihood of 

 the transformation of the latter into the former. 



(c) My third effort at a guiding generalization takes the follow- 

 ing form of statement: In attempting to find the origin of a given 

 type of animal organization foremost attention should be given to the 

 organs and parts most characteristic of the type, since the discovery of 

 the origin of these would be most decisive for the origin of the type itself. 

 Thus, could the view be fully established that mammalian hair 

 originated from epidermal sense buds like those found in amphibia, 

 this would be the strongest sort of evidence in favor of the view 

 that the ancestry of the mammals runs back to the amphibia. 

 Again, this principle would dictate that search for the origin of the 

 mollusca should be particularly promising in investigations on the 

 phylogeny of the mantle, and the shell gland. It would seem as 

 though this principle is so obvious that it should have elicited the 

 regard of every student who inquires into the relationships of larger 

 groups, at least; yet it is surprising to find how largely it has been 

 neglected. Theories of the origin of the chordata, for example, are, 

 several of them, almost wholly wanting in any serious attempt to 

 find how one of the very most characteristic things in the chordate 

 type of organization, viz., the axial skeleton, arose. Of course a 

 corollary to this principle would be that organs and parts merely 

 occasional and incidental within a given group (unless they can be 

 proven to be rudiments) can have but slight significance for the 

 affinities of groups as wholes. Thus it is really surprising that an 

 investigator with the store of learning that Gaskell possesses should 

 have so nearly set at naught his own theory by a complete disre- 

 gard of the principle. This author supposes, and goes to great pains 

 to prove, that the tubular muscles of ammocoetes are derived from 

 the veno-pericardial muscles of limulus and scorpions. And he tells 

 us this is "the strongest argument in favor of my theory." In 

 a word, the strongest argument Gaskell has in support of his view 

 as to the origin of one of the largest, most distinctly circumscribed 

 phyla of the animal kingdom, rests on an obscure group of muscles 

 found only in the larva of one small division of this phylum! 



The importance of comparative anatomy for the application of 

 this principle is likewise obvious enough. It must be the chief 

 reliance for determining the constancy of a part through a series 

 of animals. 



(d) The fourth and last principle that I here present is one 



