THE ORIGIN OF ANIMAL SPECIES 455 



or forms which are intermediate between typical 

 species. 



The causes of variability are at present very imper- 

 fectly understood, but it is probable that climate, nourish- 

 ment, and physiological activity, as well as other factors, 

 have an influence on the process. 



(5) Even though animals may be inclined to vary, 

 there is a marked tendency to inherit the characteristics 

 of their parents. Every animal bears a close re- 

 semblance to others of its kind. It is due to this 

 tendency that structural and physiological character- 

 istics once originated are perpetuated. The breeder 

 depends upon it to keep his varieties " true " to the 

 original stock. Whether or not characters acquired 

 during the lifetime of an individual are transmitted by 

 heredity to the offspring is still an unsettled problem. 

 Denial of the probability of such inheritance is a fun- 

 damental theory in the Weismannian school of evolu- 

 tionists. There seems to be no doubt, however, that 

 congenital characters are inherited. Should variations 

 appear, they are likely to be preserved to the race by 

 heredity. The essential nature of the process, as in 

 variation, is not known. To explain the phenomena of 

 heredity, Darwin proposed the theory of pangenesis, 

 which holds that particles or gemmules from all the 

 different parts of the body are collected into the repro- 

 ductive cells and through these are transmitted to the 

 offspring and help to modify the characteristics of the 

 latter. This theory has never been generally accepted. 



(6) The preservation or survival of those individuals 

 inheriting the variations which are most advantageous in 

 the struggle for existence is due to natural selection. 

 Among all the variations appearing in the individuals 

 of a race some are likely to be advantageous, others the 

 opposite. An antelope with slightly longer legs or with 



