190 HABIT AND INTELLIGENCE. [chap. 



tion — appears to stand in the closest connexion with, the 

 law of the beneficial effect of slight mixtures of race : 

 indeed, the former law is probably only a case of the 

 latter. Tliis connexion, and the whole subject of the real 

 nature of generation, can be made manifest only by study- 

 ing it among the simplest and lowest organisms. 



When the generative process is studied only among the 



highest organisms, among which it is always sexual, the 



inference is natural and inevitable that generation is 



altogether a special function; but its phenomena among 



Genera- the lowest Organisms show that it is only a modification of 



onlv a ^^® general vital process of growth and development. I 



modifiea- ■\vill here briefly recapitulate what I have said on its true 



tion of the • i i /-> • -i-, , 



general nature in the chapter on Organic Development. Any por- 



'^^^^^ tion of the structureless germinal matter of any organic 



process. . . ° jo 



species is capable of developing into a perfect individual 

 of the species, when placed under suitable circumstances ; 

 but the higher the organization, the more special the cir- 

 cumstances are required to be in order that it may so 

 develop. In hydra, a form in which there is little dis- 

 tinction between germinal matter and formed material, a 

 single small fragment, if detached, and permited to remain 

 in any place where a perfect hydra can live, will soon 

 develop into a perfect hydra. In the vast class of the 

 Protozoa, which are the simplest of all animals, and among 

 vegetable tribes of equal simplicity, propagation very gene- 

 rally takes place by means of spontaneous division into 

 parts. But among the higher forms, in which the life is 

 more centralized, and the parts more mutually dependent, 

 a portion of germinal matter, if simply detached, wiU not 

 develop into a new organism, but will die ; and, conse- 

 quently, special generative organs are set apart, in which 

 the germinal matter is prepared and supplied with nourish- 

 ment in a particular form, for the purpose of its developing 

 into a new organism. Among the lowest organic forms, 

 reproduction can scarcely be distinguished from growth. 

 Eeprodiic- Ainong aU Algae — and, indeed, throughout the vegetable 

 AicTfe. and animal kingdoms — cells give birth to cells. Among 

 the lowest Algae, in which each individual consists of but 



