106 THE LESSON OF EVOLUTION 



while others lie dormant. The Stirp contains- 

 numerous dormant organic units, which may become 

 potent in any generation. 



Professor E. Haeckel's hypothesis of " The peri- 

 genesis of the Plastidule ' >22 is as follows. He says 

 that he seeks to explain heredity by the well known 

 principle of transmitted motion. He assumes that 

 in the process of reproduction not only is the special 

 chemical composition of the plasma transmitted 

 from parent to offspring, but also a special form of 

 molecular motion, which belongs to its physico- 

 chemical nature. The plasma is supposed to be 

 composed of plastidules, each of which is probably 

 surrounded by water. Heredity is the transmission 

 of plastidule motion ; whereas adaptation is change 

 of plastidule motion. This motion, he says, may, in 

 its general aspects, be conceived of as a ramified 

 wave-action. 



In 1880 Dr. W. Roux proposed to explain variation 

 by a process of intercellular natural selection. He 

 supposed that, owing to pathological or other con- 

 ditions, a group of cells would not be so well 

 nourished as their neighbours, which would, there- 

 fore, take a new lead in the development of the 

 organ. This has been called Intra-selection , or the 

 selection of organs. 



Professor A. Weismann published, in 1883, his 

 well known hypothesis that heredity is due to the 

 continuity of the germ-plasm. 23 Protoplasm is 



22 Schopfungsgeschichte, 1876, p. 200: also "Nature," vol. 

 xiv, p. 235. 



23 " Essays on Heredity." English translation published 

 by the Clarendon Press. 



