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 
archzopteryx 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 
