IV EFFECTS OF TROPICAL CLIMATE 101 



tion given to the evolution could the artificially produced 

 characters possibly be maintained. In Schiibeler's experi- 

 ments, by attention to the necessary requirements, artificial 

 characters were to a certain degree maintained. But nature 

 alone can completely satisfy these requirements. If we had 

 in our artificial experiments command of the measureless 

 periods of time which have been employed by nature, we 

 should be able perhaps in many cases to imitate artificially, 

 or even to surpass, her success in producing permanent 

 modification. 



The facts, however, which are brought to light by study 

 of the gradual transformation in plants and animals seen in 

 passing from north to south, or from high to low elevations, 

 afford the clearest proof that climatic conditions have pro- 

 moted, i.e. have helped to produce that transformation ; and 

 the gradual transformation of extinct plant and animal forms 

 also points in the most definite way to such external causes. 



This gradual transformation is in many ways so obvious 

 that its explanation becomes of secondary importance com- 

 pared with the problem of the causes of the separation of the 

 organic chain into species. That the chain was originally 

 continuous is proved by the fact that it can be actually traced 

 without interruption between widely separated limits among 

 extinct organic forms. 



The tropical climate, however, certainly has exercised, 

 sometimes in a very short time, a direct influence on particular 

 characters of animals, especially on the integument, on the 

 thickness of the coat of hair, on the nature of the wool, etc. 

 This is demonstrated by authentic instances. 



In India our races of dogs after only two generations lose 

 their distinguishing characteristics ; thus, for example, in 

 pointers the nostrils become more contracted, the snout more 

 pointed, the size smaller, the limbs more slender (Everest). 

 On the Guinea coast the ears of dogs become long and stiff, 



