Nature of Heredity. 337 



these modifications have in their turn been influencing the germ- 

 cells by internal equilibration, and that the germ-cells will store up 

 the stimuli which they receive so as to be capable of producing a 

 kind of after-effect upon the developing organisms even after the 

 original external stimulus has been completely removed. The amount 

 of this after-effect, if we may so term it. will be proportional tot 

 the length of time for which the original stimulus acted. 



We cannot, therefore, as we have already observed, expect a 

 character which has been acquired in a single generation, and has 

 had little time toi influence the germ-cells, to make a sufficiently deep 

 impression upon them to secure its own transmission as an after- 

 effect, and hence the scarcity of evidence for the transmission of 

 acquired characters, although, as we have already seen, there are 

 exceptional cases in which suddenly acquired characters make such 

 a deep impression upon the germ-cells as to be transmitted to the 

 next generation. It is. however, to quote the words of Darwin. 

 " generally necessary that an organism should be exposed during 

 several generations to changed conditions or habits, in order that 

 any modification thus acquired should appear in the offspring.''*) 



The development of a complex organism may then be looked 

 upon as consisting mainly of a series of superposed after-effects, 

 which continue to shew themselves long after the originating stimuli 

 have been removed. 



The various stages in the ontogeiiy are also probably deeply 

 fixed by mere repetition, for the more frequent the occurrence of 

 any particular somatic condition the deeper will be the impression 

 which it makes upon the germ-cells and the more difficult will it 

 be to remove that impression. 



This method of looking upon the ontogeny or life history of 

 an individual as a series of after-effects, first suggested by Detmer, 

 is. as far as I can see. the only rational wax oi explaining the 

 Biogenetic Law, or, as it is sometimes termed, the Law of Recapitula- 

 tion, in accordance with which organisms tend to repeat, in their 

 own development, the ancestral history or phvlogenv of the species 

 to which they belong. 



(6). AMPHIMLXIS OR SEXUAL REPRODUCTION. 



The phenomena of heredity are usually greatly complicated by 

 the process of sexual reproduction or amphimixis. This consists 

 essentially in the union or conjugation of two germ-cells, usually 

 derived from separate individuals, to form a third cell known as 

 the zygote, which develops into a new individual. 



If has ,been shewn that the essence of this process of conjuga- 

 tion lies in the union of the so-called pronuclei of the male and 

 female gametes or germ-cells within the protoplasmic body of the 



* Animals and Plants under Domestication, Vul. II., p. 381). 



