316 THE CELL 



tion). If the ova are fertilised after twenty-five hours, some 

 develop normally, and a not inconsiderable number irregularly, 

 in consequence of multiple fertilisation, whilst a small number re- 

 main unaffected. 



The results obtained with Echinoderm ova seem to me to offer an 

 explanation of the fact, that domesticated animal and vegetable 

 species are generally more easily crossed than nearly related species 

 in the state of nature. The entire constitution seems to be altered 

 and rendered less stable by domestication. The changes are most 

 evident in the sexual products, since the generative apparatus is 

 sympathetically affected by any variations which take place in the 

 body. 



In self-fertilisation, as in hybridisation, sexual affinity is in- 

 fluenced by the environment. Darwin (VII. 8) has pointed out, 

 that Eschscholtzia californica cannot be induced to fertilise itself in 

 Brazil, whilst it can in England ; moreover, if seeds from England 

 are taken back to Brazil, they quickly become useless for self- 

 fertilisation. Further, various individuals behave in different 

 manners. Just as in Echinoderms, in which some of the ova of an 

 ovary may be fertilised with foreign spermatozoa, and others not, 

 so we find experimentally, that some individuals of Reseda odorata 

 can fertilise themselves whilst others cannot. In a similar manner 

 we must attribute to individual differences of the egg-cells of an 

 ovule the circumstance that in many plants far fewer seeds are 

 produced by self-fertilisation and hybridisation than by normal 

 fertilisation. A certain number of egg-cells either are not receptive 

 to the foreign pollen, or if they do become fertilised, die prema- 

 turely. 



Recapitulation and attempted Explanations. If we now 

 review the facts described in the last chapter, there can be no 

 doubt but that the necessity of fertilisation of the sexual cells and 

 sexual affinity, which is closely connected with it, are extremely 

 complicated, vital phenomena. The factors which are influential 

 here are beyond our knowledge. Many circumstances seem to 

 point to the fact, that the conditions, under which the egg-cells 

 are able to develop either parthenogenetically or in connection 

 with a sperm-cell, must be sought for in small differences of 

 molecular organisation. Similarly, we can only explain the facts, 

 that sometimes self-fertilisation and cross-fertilisation are possible, 

 and at others not, that the egg-cells of the same individual often 

 behave differently during self-fertilisation and cross-fertilisation, 



