REPRODUCTION AND HEREDITY 257 



stock which, however, mostly consists of only two or three 

 individuals. Only very rarely have I succeeded in breeding large 

 stocks of four and sometimes five individuals. This state, how- 

 ever, does not last long, for the buds soon separate from the 

 maternal body and commence an independent existence. 



Numerous species of marine jelly fishes form, by gemmation, 

 enormous animal states which frequently consist of hundreds 

 of thousands of individuals remaining permanently united. But 

 as stocks of such immense size and with such enormous power 

 of reproduction could not exist long without the necessary 

 mechanical support, these animalcules jointly construct a firm 

 skeleton of carbonate of lime, or, in some species, of a horn- 

 like substance. 



The most magnificent instance of such animal colonies is 

 doubtless that of the corals whose restless labour makes new 

 islands and reefs rise from the ocean-bed and played a most 

 important part in the early life of the earth. 



It seems that asexiial reproduction cannot proceed uninter- 

 ruptedly in any of the higher species, for after a series of genera- 

 tions it is always once more replaced by sexual reproduction. We 

 are probably right in assuming that the faculty of gemmation and 

 fission was only a very late acquisition of the higher multicellular 

 animals, and that it is an adaptation to the special conditions 

 of their life. The original method however is reproduction by 

 germ-cells. In the metazoans, therefore^ conditions would on this 

 assumption be opposite to those in the protozoans, in whom we 

 saw asexual reproduction to be the original method, while sexual 

 union was doubtless only evolved at a much later period of their 

 phylogeny. That with an assumption of this kind we are only 

 dealing with probabilities and guesses and that we are completely 

 deserted by experience needs no particular mention. 



The inclination to asexual reproduction is usually increased 

 by favourable conditions of life and abundant food ; want of food, 

 on the other side, and other influences which threaten their 

 existence induce organisms to proceed to sexual reproduction. 

 It is only necessary to restrict the amount of food of the Hydra 

 in order to observe soon afterwards the formation of sex- 

 products. 



Whether exclusive asexual reproduction is not possible in 

 17 



