THE VITAL PROPERTIES OF THE CELL 313 



tion does take place, the seedlings may be either weakly or 

 vigorous. Hybrids of widely different species are often very 

 delicate, especially in youth, so that it is difficult to rear them. 

 On the other hand, hybrids of 'nearly related species are usually 

 uncommonly luxuriant and vigorous ; they are distinguished by 

 their size, rapidity of growth, early blooming, long life, wealth of 

 blossoms, strong powers of multiplying, the unusual size of 

 individual organs, and similar properties. 



Hybrids of different species develop a smaller quantity of 

 normal pollen grains in their anthers than plants of pure descent; 

 frequently they produce neither pollen nor ovules. In hybrids 

 of nearly related species, this weakening of the sexual reproduc- 

 tive powers is not usually to be observed. 



As a genei'al rule, the closer the relationship of the parents, 

 and the greater their sexual affinity, the better does their hybrid 

 product thrive. In individual cases it may get on even better 

 than that of a normally fertilised ovum. For example, when egg- 

 cells of Nicotiana rustica are crossed with pollen of N. Califoriiica, 

 a plant is produced which, as regards height, stands to its parents 

 in the ratio of 228 : 100 (Hensen VII. 18). 



y. The Influence of the Environment upon Sexual 

 Affinity. We have seen in our experiments upon self-fertilisa- 

 tion and hybridisation, that the sexual affinity of the egg and 

 sperm-cells is a factor which cannot be reckoned upon with cer- 

 tainty, and with which a series of the most different resulting 

 phenomena is connected; such as fertilisation or non -fertilisation, 

 development which has been prematurely hindered and weakened, 

 or which has been rendered more vigorous, etc. We shall find, 

 however, that the phenomenon of sexual affinity is still more 

 complicated by the fact that in many cases it may be influenced 

 by external circumstances. 



Most peculiar facts concerning hybridisation have been dis- 

 covered by means of experimental researches upon certain 

 Echinoderms (VII. 20). The unfertilised ova are naked, but 

 in spite of this, fertilisation does not usually take place when 

 spermatozoa, which are of nearly related species, and are exactly 

 similar in appearance, are placed in their neighbourhood, although 

 these latter settle upon the surface of the ova, and make boring 

 movements. In this case the non-fertilisation can only be ex- 

 plained by imagining, that the ovum, if I may use the expression, 

 refuses to admit the unsuitable spermatozoon. 



