364 THE POPULAR SCIENCE MONTHLY. 



SPECULATIVE ZOOLOGY. 



By Professor W. K. BEOOKS, 



OF JOHNS HOPKINS UNIVERSITY. 

 II. 



WE will now examine our various sources of information, in order 

 to see how far the evidence which they furnish can be used to 

 establish phylogenies. 



Comparative anatomy might not at first sight be expected to yield 

 much of this kind of evidence, for the only animals which we can 

 study thoroughly are those recent ones which have diverged very 

 widely from their remote ancestors ; and, while it is true that the study 

 of the structure of living animals does not furnish direct evidence, 

 the doctrine of homology supplies a means of sifting out, by general 

 comparisons, what has been recently acquired from what is more 

 deep seated, and of thus arranging animals in a series of groups of 

 greater and greater extent and less and less contact. This classifica- 

 tion of animals upon morphological grounds is essentially phylogenetic, 

 for the difference between a system of converging lines and a sys- 

 tem of more and more inclusive definitions is simply a difference in 

 the manner of expression ; nor can it be said that the one method 

 assumes the disputed point, genetic relationship, any more than the 

 other, for the naturalist who believes that classification is not simply a 

 matter of convenience, but that there is one natural system, and that, 

 according to this system, living things fall into a few great groups, 

 each of which is characterized by certain general features, and that 

 each of these groups is divided into smaller groups distinguished in a 

 similar way, and these again into smaller groups, and so on, tacitly 

 assumes that the natural system of classification or relationship is what 

 we should expect it to be if the theory of descent with modification is 

 true. 



If there is a natural " systematic classification," it must be exactly the 

 same as a phylogenetic tree, and the idea of descent is no more essential 

 in the one case than it is in the other ; they are simply different ways 

 of expressing the same thing, the relationship of living things, and 

 neither of them involves more than the other any particular interpre- 

 tation of the word relationship ; nor can it be said that, while the one 

 method assumes that the larger trunks of the system have at some 

 time been embodied in actual organisms, the second method allows us 

 to believe that these groups are purely ideal, for, although this latter 

 conception may have been defensible to some extent in the early days 

 of morphological science, the progress of discovery has shown that 

 even nowadays animals exist in which the characteristics of a branch 



