88 Evolution and Adaptation 



then we are only removing the explanation one step farther 

 back. If this original group has come from numerous species 

 of a still older group, and this, in turn, from an older one 

 still, then we must go back to the first forms of life that ap- 

 peared on the globe, and suppose that the individuals of these 

 primitive forms are the originals of the species that we find 

 living to-day. For instance, it is thinkable that each species 

 of vertebrate arose from a single group of the earliest forms 

 of life that appeared on the surface of the earth. If this 

 were the case, there must have been as many different kinds 

 of species of the original group as there are species alive at 

 the present time, and throughout all the past. This view finds 

 no support from our knowledge of fossil remains, and, al- 

 though it may be admitted that this knowledge is very in- 

 complete, yet, if the process of evolution had taken place as 

 sketched out above, we should expect, at least, to have found 

 some traces of it amongst fossil forms. Since this question 

 is an historical one, we can, at best, only expect to decide 

 which of all the possible suggestions is the more probable. 

 We conclude, then, that it is more probable that the verte- 

 brates, the mollusks, the insects, the crustaceans, the annelids, 

 the ccelenterates, and the sponges, etc., have come each from 

 a, single original species. Their resemblances are due to a 

 cofrrmon inheritance from a COT&niQn_ancestral species. Even 

 if it be probable that at the time when the group of verte- 

 brates arose from a single species, there were in existence 

 other closely related species, yet we must suppose, if we 

 adhere to our point of view, that these other related species 

 have had nothing to do with the group of vertebrates, but that 

 they have died out. Moreover, we must suppose that each 

 order, each class of vertebrate, has come from a single origi- 

 nal species ; each family has had a similar origin, as well as 

 each genus, but, of course, at different periods of time. Let 

 us not shrink from carrying this principle to its most extreme 

 point, for, unless the principle is absolutely true, then our 



