158 THE MAIN CURRENTS OF ZOOLOGY 



how it will behave in development. Two separate 

 masses of protoplasm differ in detail, as to capabili- 

 ties and potentialities, according to the experiences 

 through which they have passed, and no two will be 

 absolutely identical. 



We have seen that variation and heredity are the 

 two primary factors of evolution. The way in which 

 Weismann accounts for variation among the higher 

 animals is both ingenious and interesting. In all 

 higher organisms the sexes are separate and repro- 

 duction of their kind involves the union of germinal 

 elements from both parents. 



As previously stated, the germinal elements in-' 

 volved are ovules and pollen of plants and eggs and 

 sperms of animals. In animals, the egg bears all 

 hereditary qualities from the maternal side, and the 

 sperm those from the paternal side. The intimate 

 mixture of these in fertilization gives great possibil- 

 ities of variations arising from the different com- 

 binations and permutations of the vital units within 

 the germ-plasm. 



The variations once started will be fostered by 

 natural selection. 



It is now evident that if we follow Weismann's 

 conclusions logically, there can be no inheritance of 



