340 THE HISTORY OF CREATION. 
quickly die out, while the most divergent forms remain and 
reproduce themselves as distinct “new species.” In accord- 
ance with this, we in fact no longer find forms of transition 
leading to those groups which are becoming extinct, as, 
for example, among birds, are the ostriches; and among 
mammals, the elephants, giraffes, Semi-apes, Edentata, and 
ornithorhyncus. The groups of forms approaching their 
extinction no longer produce new varieties, and naturally 
the species are what is called “ good,” that is, the species 
are distinctly different from one another. But in those 
animal groups where development and progress are still 
active, where the existing species deviate into many new 
species by the formation of new varieties, we find an 
abundance of transition forms which cause the greatest 
difficulties to systematic naturalists. This is the case, for 
example, among birds with the finches; among mammals 
with most of the rodents (more especially with those of the 
mouse and. rat kind), with a number of the ruminants 
and with genuine apes, more especially with the South 
American forms (Cebus), and many others. The continual 
development of species by the formation of new varieties 
here produces a mass of intermediate forms which connect 
the so-called “good” species, which efface their boundaries, 
and render their sharp specific distinction completely 
illusory. 
_ The reason that this nevertheless does not cause a com- 
plete confusion of forms, nor a universal chaos in the struc- 
ture of animals and vegetables, lies simply in the fact 
that there is a continual counteraction at work between 
progressive adaptation on the one hand, and the retentive 
power of inheritance on the other hand. The degree of 
