ABUNDANCE OF HYPOTHETICAL STAGES. 151 



ape are descended from a third form not living to- 

 day, but which is represented by the stage which 

 the two embryos have in common just before birth. 

 Now this hypothetical form was neither man nor 

 monkey, but midway between them. If we should 

 find his bones in the rocks we might not unlikely 

 call him an ape, but if we should know more about 

 him we should find that he did not resemble any 

 animal living to-day. Or again, man has not descend- 

 ed from an animal like our modern fish, but both 

 man and our fishes have descended from a third 

 intermediate form. This form doubtless lived in 

 the water and would probably be called a fish were 

 we to discover its remains in the rocks, but it was in 

 reality very different from our modern fishes. For 

 a time our fishes and mammals travelled together, 

 but soon one group took one direction and the 

 other another, both continuing to develop, getting 

 farther and farther from each other and from this 

 common ancestor. And so everywhere. The types 

 of animals which were the common ancestors of our 

 existing groups are all extinct. Embryology shows 

 us that if we wish to trace the past history of ani- 

 mals, it must be mostly done through hypothetical 

 forms, which have existed in the past but are now 

 extinct. 



The extent to which this principle is true, is excel- 

 lently illustrated by a tentative ancestry of the 

 vertebrates as taught by one of the ablest of modern 

 scientists. If one takes the trouble to refer to vol. ii. 

 of Balfour's "Comparative Embryology," it will be 

 seen that he derives the mammals from the earliest 



