SPECULATIVE ZOOLOGY. 203 



than descent from a common ancestor, a, different from the common 

 ancestor b of D, E, and F, and all may be the descendants of G, in 

 the way shown by the heavy lines, or A may be the descendant of I ; 

 B of K ; C of G, etc. If we were to attempt to indicate all the 

 possible ways in which the six living species, A, B, C, D, E, and F, 

 may be related to the fossil M, the diagram would become a confused 

 mass of lines, and we have pointed out enough to show that, in a very 

 simple case, where there are only two living genera and only six 

 species, the attempt to follow them back only two stages to a common 

 ancestor leads to so many possible systems of relationship that there 

 is a very great chance against the truthfulness of any particular one, 

 and we may fairly ask whether the attempt to express the relation- 

 ships of animals in a tree-like classification can have any scientific 

 value if the chances against its correctness are so very great. At first 

 sight it may seem as if no good could be expected from this sort of 

 speculation, and we may feel inclined to condemn the construction of 

 phylogenetic trees as unscientific ; but a little examination will show 

 that all the lines in the diagram agree in one important particular, 

 and trace the recent animals, A, B, C, D, E, and F, back to a remote 

 common ancestor with a general resemblance to M. This, after all, is 

 the essential thing, the gist of the whole matter, for the precise line 

 of descent has no more scientific interest than the exact pedigree of 

 each person would have to the anthropologist of our illustration. 

 Such an exact pedigree would have a certain value as a bit of specific 

 information, but the general evidence is of such a character that it is 

 more logical to accept the conclusion than it is to reject it, and it is 

 as truly scientific as the conclusion of our anthropologist. 



We find that living things are related to each other in a pecul- 

 iar way, which can be explained upon the assumption that they are 

 the modified descendants of more ancient generalized forms, with 

 wider relationships, and this assumption can be readily expressed in 

 the form of a phylogenetic tree. We find, too, that so far as the 

 higher groups of vertebrates, the mammals, reptiles, and birds, are 

 concerned groups which are of comparatively recent appearance, like 

 the last races of immigrants in our imaginary case the fossil forms 

 which we meet with are such as our assumption would lead us to 

 expect. The presumption is, therefore, very great that the genetic 

 relations of living things may be expressed with general accuracy by 

 a phylogenetic tree, although the chances of minute accuracy of detail 

 in favor of any particular tree which is drawn up from paleonto- 

 logical evidence are very slight. This lack of minute accuracy 

 can not be urged as an objection to all attempts at following out, in 

 a general way, the lines of evolution of our present groups of ani- 

 mals, according to the best evidence which is attainable, and we must 

 remember that only a very small part of this evidence is furnished 

 by paleontology. If no fossils were known, the facts of comparative 



