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 ia 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 difierent 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 



