THE VITAL PROPERTIES OF THE CELL 317 



that the need for fertilisation and parthenogenesis, or the success 

 of self-fertilisation and cross-fertilisation, may often be influenced 

 by external circumstances, and that the well-being of the pro- 

 ducts of generation is dependent upon the mode of fertilisation, 

 by the presence of these same differences of molecular organisa- 

 tion. 



What now must be the molecular organisation of the sexual cells 

 which renders them suitable for the purposes of fertilisation ? 

 Some help towards solving this problem may be obtained by com- 

 paring the phenomena of self- fertilisation and bastard fertilisation 

 with normal fertilisation. 



As is evident from numerous observations, the result of fertilisa- 

 tion is essentially determined by the degree of relationship which 

 the male and female sexual cells bear to one another. The 

 process of fertilisation is prejudiced by a relationship which is 

 either too near or too distant ; or, as we may express it, by a too 

 great similarity, or a too great difference. Either the sexual cells 

 do not unite at all, since they exhibit no sexual affinity towards 

 each other, or the mixed product of both, i.e. the embryo pro- 

 duced by fertilisation, is unable to develop in a normal manner. In 

 the latter case the embryo may either die during the first stages of 

 development, or it may live as a weakly product ; or further, this 

 weakly product, owing to the destruction of its capacity for re- 

 production, may be useless for the preservation of the species. 

 In all cases the product of reproduction thrives best when the 

 generative individuals, and consequently their sexual cells, differ 

 only slightly in their constitution and organisation. 



Darwin (VII. 8) rendered science a great service when, by means 

 of his extensive experiments and investigations, he laid the 

 foundations of this knowledge, and first clearly formulated these 

 theories. I will quote three of his sentences : " The crossing of 

 forms only slightly differentiated favours the vigour and fertility 

 of their offspring . . . and slight changes in the conditions 

 of life add to the vigour and fertility of all organic beings, 

 whilst greater changes are often injurious." The act of crossing 

 in itself has no beneficial effect, but "the advantages of cross- 

 fertilisation depend on the sexual elements of the parents having 

 become in some degree differentiated by the exposure of their 

 progenitors to different conditions, or from their having inter- 

 crossed with individuals thus exposed, or lastly from what we 

 call in our ignorance ' spontaneous variation. ' " The need of 



