42 Evolution and Adaptation 



one, that it itself goes out of existence, being exterminated 

 by the new form. There may be in fact no relation what- 

 soever between the birth of a new group and the extermina- 

 tion of an old one. 



On the transmutation theory we should expect to find not 

 only a sequence of forms, beginning with the simplest and 

 culminating with the more complex, but also, in the beginning 

 of each new group, forms more or less intermediate in 

 structure. It is claimed by all paleontologists that such 

 forms are really found. For example, transitional forms 

 between the fishes and the amphibia are found in the group 

 of dipnoans, or lung-fishes, a few of which have survived to 

 the present day. There are many fossil forms that have 

 characters between those of amphibians and reptiles, which 

 if not the immediate ancestors of the reptiles, yet show 

 that at the time when this group is supposed to have 

 arisen intermediate forms were in existence. The famous 

 archaeopteryx remains have been already referred to above, 

 and it appears in this case that we have not only an inter- 

 mediate form, but possibly a transitional one. In the group 

 of mammals we find that the first forms to appear were the 

 marsupials, which are undoubtedly primitive members of the 

 group. 



The most convincing evidence of transmutation is found in 

 certain series of forms that appear quite complete. The 

 evolution of the horse series is the most often cited. As this 

 case will be discussed a little later, we need not go into it 

 fully here. It will suffice to point out that a continuous 

 series of forms has been found, that connect the living 

 horses having a single toe through three-toed, with the five- 

 toed horses. Moreover, and this is important, this series 

 shows a transformation not only in one set of structures, but 

 in all other structures. The fossil horses with three toes are 

 found in the higher geological layers, and those with more 

 toes in the deeper layers progressively. In some cases, at 



