198 PHYSIOLOGY CHAP. 



approach, unite, and fuse together. The success of the fertilisa- 

 tion depends on the degree of affinity existing between the two 

 sexual cells, not upon difference but upon similarity of their 

 intimate organisation. When the relationship is too near or too 

 remote, that is, when the resemblance or the dissimilarity is too 

 slight, or too great, the success of the fertilisation is compromised 

 in an absolute or a relative manner. " The crossing of forms," 

 wrote Darwin, " which have been exposed to conditions of life a 

 little different, or which have undergone variations, favours the 

 vital energy and fertility of their descendants, while more con- 

 siderable variations are often unfavourable." 



To illustrate these general propositions with a particular 

 example I think it will be useful to refer to the most recent 

 experiments carried out at the zoological station of Naples by 

 von Dungern (1901) on echinoderms. When the ova and 

 spermatozoa of the star-fish (asterias) and the sea-urchin (echinus) 

 are placed in a vessel of sea water, it is seen that the spermatozoa 

 of one species are attracted by the ova of the other species 

 promiscuously, without, however, fertilisation and the generation 

 of bastard products taking place. According to the observations 

 of von Dewitz, confirmed by von Dungern, the presence of the 

 ova of one species exercises on the spermatozoa of another species 

 a stimulating influence, which makes them more lively, but at the 

 same time changes the direction of their movements, so that they 

 slide with their heads oblique on the surface of the ovum in a 

 manner that does not enable them to penetrate and pass into the 

 yolk. On the spermatozoa of the same species, however, the ova 

 exercise a certain inhibitory and directing action, so as to enable 

 them to place their heads in the proper position that they may 

 penetrate into the yolk and fertilise them. This then is a com- 

 plex process determined by forces difficult to reduce to a simple 

 chematactic action. 



According to the new researches of von Dungern, fertile 

 pairing between the sexual elements of different species may be 

 also hindered by two other mechanisms. From the ova of the 

 star-fish he isolated a substance resistant to heat, which proves 

 poisonous in very small doses to the spermatozoa of the sea- 

 urchin, but not to those of the star-fish : in the ova of the sea- 

 urchin, on the other hand, such a substance, poisonous to the 

 spermatozoa of the star-fish, was absent ; the spermatozoa may 

 make contact with the ova of the echinus without dying, but 

 cannot penetrate and fertilise them, because they become agglu- 

 tinated in the gelatinous coverings which wrap them round. 



These results appear the more interesting when one recalls 

 the modern researches on immunity inaugurated by Ehrlich, 

 from which the conclusion is drawn that the animal organism is 

 endowed with the capacity of forming, under certain circumstances, 



