Heredity and the Origin of Variations. 153 



of the necessary factors in distinc't varieties, are again 

 rendered possible when these varieties interbreed. 



On this hypothesis of organic combination, to which we 

 shall recur in the chapter on " Organic Evolution," the 

 varied forms of animal life are the outcome of definite 

 organic products with definite organic structure, analogous 

 to the definite chemical compounds with definite crystalline 

 and molecular structure ; and the analogy between the 

 regeneration of hydra and the reconstruction of a crystal 

 is carried on a step further. I do not say that I am myself 

 at present prepared to adopt the hypothesis, at least in 

 this crude form ; but it is, perhaps, worth a passing con- 

 sideration. Its connection with Mr. Herbert Spencer's 

 doctrine of physiological units is obvious. The analogy 

 there is with crystallization ; here it is with chemical 

 combination. 



We must now return to the point which gave rise to 

 this digression, and repeat that mere hereditary com- 

 mixture in the union of ovum and sperm cannot give rise 

 to new characters or raise existing structures (1) where 

 there is free intercrossing beyond the mean of the species, 

 and (2) where there is rigorous elimination beyond the 

 existing maximum of the species. Variations beyond this 

 existing maximum must be due to some other cause. 



Professor Weismann has suggested, as a cause of varia- 

 tion, the extrusion of the polar cells from the ovum. It 

 has before been mentioned that, generally previous to 

 fertilization, the ripe ovum buds off two minute polar 

 bodies. The nucleus of the ovum divides, and one half is 

 extruded in the first polar cell ; the nucleus then (except in 

 parthenogenetic * forms, where there is no union of ovum 

 and sperm) again divides, and a second polar cell is extruded. 

 In accordance with his special view of the absolute dis- 

 tinction between the body-plasm and the germ-plasm, the 

 first polar cell is formed to carry off the body-plasm of the 



* One parthenogenetic form— the drone — has been shown by Blochmann 

 to extrude a second polar cell. This observation is iu serious opposition to 

 Dr. Weismann's theory, 



