SPECULATIVE ZOOLOGY. 20l 



cover, and the circumstantial evidence converges in such a way as to 

 give in this case every assurance of substantial accuracy. 



There is not a single wild species of animal which can be traced, 

 by direct unbroken pedigree, to an ancestor belonging to a different 

 genus, and no zoologist has any hope of ever obtaining anything 

 more than circumstantial evidence of such a pedigree ; but we can 

 hardly overestimate the vast and increasing stores of evidence in favor 

 of evolution which are yielded by the structural, geographical, and 

 chronological relations between the fauna of the present day and that 

 of the past. It is true that all this evidence is circumstantial, and, al- 

 though it renders the theory of evolution vastly more probable than 

 any other explanation of the origin of species, it still leaves a possi- 

 bility that some other explanation may be the true one. Although 

 the investigator who is fully acquainted with all the evidence may 

 feel justified in ignoring this possibility, it still exists. 



If the evidence which we have is so circumstantial that it does not 

 amount to absolute proof, it is clear that, even if we fully believe in 

 evolution, we can not hope to trace, with anything like minute ac- 

 curacy, the past history of any particular form of life ; but perhaps an 

 illustration may help to make this clearer : 



Let the dots A, B, and C (Fig. 1), represent a number of recent spe- 

 cies, each of which has distinctive characteristics of its own, together 

 with other characteristics which are common to all; and let D, E, and 

 F be another set of species related to each other in the same way; and 

 suppose that certain of the common characteristics of D, E, and F are 

 also common to A, B, and C, while others distinguish the one set as a 

 whole from the other as a whole. According to the theory of evolu- 

 tion, we believe that A, B, and C are the descendants of an ancestor 

 from whom they inherit all that they have in common ; and that D, 

 E, and F are related to each other in the same way; that the common 

 ancestor of all the forms in the first group had, together with distinct- 

 ive characteristics of its own, certain other characteristics which it 

 shared with the ancestor of the forms in the second group, and that 

 this similarity was due to inheritance from a still more remote ances- 

 tor common to both. This system of relationship might be expressed 



by a phylogenetic tree, like that which is shown by dotted lines 



in the diagram, with six ultimate ramules, two large branches, and a 

 common stem, G. Now, suppose that we discover, in a recent geolog- 

 ical formation, a fossil form, M, which resembles A, B, C, D, E, and F 

 in all the features which they have in common. It is possible that 

 this fossil is the form G, from which A, B, C, D, E, and F are de- 

 scended, but it is not probable that this is the case, for the analogy of 

 recent species compels us to believe that the fossil M was one of sev- 

 eral closely related species, G, H, I, K, and L, any one of which may 

 have been the ancestor of the recent forms, and, as M is only one out 

 of several species, the chances are that it is not the root G from which 



