HEREDITY 



71 



Elasmobranch and Bird embryos. But this need not lead 

 us to deny the general resemblance. 



Moreover, the individual life history is much shortened 

 compared with that of the race. Not merely does the one 

 take place in days, while the other has progressed through 

 ages, but stages are often skipped, and short cuts are dis- 

 covered. And again, many young animals, especially those 

 "larvfe" which are very unlike their parents, often exhibit 

 characters which are secondary adaptations to modes of life 

 of which their ancestors had probably no experience. In 

 short, the individual's recapitulation of racial history is gen- 

 eral, but not precise. 



But we do not understand how the recapitulation is sustained. Has 

 the protoplasm of the embryo some unconscious memory of the past? 

 Have the protoplasmic molecules, as Ha;ckel puts it, learned long since 

 some rhythmic dance which they cannot forget ? And, to what extent 

 must there be similarity of external conditions if the recapitulation, "the 

 perigenesis of the plastidules," is to be sustained? For a careful state- 

 ment of the problem, the student would do well to read the late Pro- 

 fessor Milnes Marshall's British Association address on RECAPITULA- 

 TION, now published in his collected papers. 



(4) Organic Continuity between Generations. — Heredity. 

 — Every one knows that like tends to beget like, that off- 

 spring resemble their parents, and soinetiines their ancestors 

 (atavism). Not only are the general characteristics trans- 

 mitted, but minute features, idiosyncrasies, pathological 

 conditions, innate or congenital in the parents, may be 

 transmitted to the offspring. 



Many attempts have been made to explain this, but the 

 first suggestion with any scientific pretensions was that the 

 reproductive cells, which may become offspring, consist of 

 samples accumulated from the different parts of the body. 



This was a very old idea, but Herbert Spencer and 

 Charles Darwin gave it new life. According to Darwin's 

 " provisional hypothesis of pangenesis," the reproductive 

 cells accuinulate gemmules liberated from all parts of the 

 body. In development these gemmules help to give rise to 

 parts like those from which they originated. This hypo- 

 thesis has been repeatedly modified, but, except in the gen- 

 eral sense that the body may influence its reproductive cells, 

 "pangenesis" is discredited by inost biologists. 



The idea which is now accepted with general favour is, 



