454 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



that system of cells called the nervous system, which 

 has been specialized through use and natural selection 

 to receive impressions from without, and to transmit 

 them to such parts of the organism as, are capable of 

 receiving them, or whether it is transmitted through 

 other media, as in lower animals and in plants which 

 possess no such system, is unknown. The only cells 

 which can retain the entire record in the higher ani- 

 mals are the reproductive cells. In the lower animals 

 and plants it is well known that germ-plasma is not 

 confined to reproductive organs, but is widely dissemi- 

 nated throughout the organism. In some forms it 

 seems that all of the sarcode is capable of reproduction. 



This is the logical result of the considerations 

 which have occupied the preceding pages, and is the 

 carrying out of the bathmism theory of heredity, of 

 which I have given hitherto only the bare outline. 



Since Darwin, successive contributions have been 

 made to the theory of heredity in its relation to evolu- 

 tion. In 1868 and 1871 the present writer advanced 

 the dynamic hypothesis, but made no attempt to ex- 

 plain the mode of conveyance of dynamic impressions 

 and modifications to the germ-cells. The theory of 

 "perigenesis," proposed by Haeckel in 1873, is of the 

 same character, and is deficient in the same way. The 

 modified pangenesis theory of Brooks, published in 

 I883, 1 attempts to supply the defect found in the pre- 

 vious conceptions, but does so by assuming with Dar- 

 win the intermediation of gemmules, a hypothesis to 

 which sufficient objection has been made by Galton 

 and others. Brooks's theory also fails to admit the 

 origin of variations through mechanical stresses, al- 

 though he seeks for the origin of gemmules through 



1 The Law of Heredity, Baltimore, 1883, p. 80. 



