6 DEVELOPMENT AND HEREDITY. 



omitted above for simplicity's sake), that each ani- 

 mal arises from the fusion of the germ-plasms of 

 two individuals, and this fused germ-plasm from 

 which the animal grows and which he transmits, 

 is slightly different from either original germ-plasm 

 — being a cross between the two. Therefore the 

 animal differs slightly from either parent. This is 

 the origin of variation. The union of the two 

 germ-plasms determines all the characters of the 

 animal which is to arise from it, and all the ten- 

 dencies which he will transmit to his offspring. 

 Whatever happens to the animal during his life- 

 time cannot change these tendencies ; for they 

 are inherent in the germ-plasm, and the composi- 

 tion of the germ-plasm became fixed by the union 

 of the two germ-plasms of the parents. Thus the 

 transmission of acquired characters is impossible. 



The fundamental truths on which this theory is 

 based are too well known to need repetition. The 

 theory itself is at once seen to involve the most 

 extreme phase of the natural selection theory, dis- 

 claiming everything like a causal relation between 

 environment and favourable variation ; and necessi- 

 tating the action of natural selection on the most 

 minute variations, as, for instance, those necessary 

 to produce the human eye. There is another 

 remarkable part of the theory of variation which- 



