THE VITAL PROPERTIES OF THE CELL 311 



Although the only difference between Anagallis arvensis and A. 

 ccerulea is in the colour of their blossoms, they cannot be induced 

 to fertilise each other. No hybrids have been obtained from 

 apple and pear-trees, or from Primula officinalis and P. elatior ; 

 whilst, on the other hand, hybrids have been successfully obtained 

 between species which belong to different orders, such as Lychnis 

 and Silene, Rhododendron and Azalea, etc. 



Sachs says : " The absence of correspondence between sexual 

 affinity and systematic relationship is shown in a more striking 

 manner, in that occasionally varieties of the same species are 

 either quite unable to fertilise each other, or can only do so to a 

 partial extent ; thus Silene inflata var. alpina cannot conjugate with 

 var. angusiifolia, nor var. latifolia with var. litoralis, and so on." 



In both the animal and the vegetable kingdoms we find certain 

 orders the species of which can be easily crossed, whilst there are 

 others whose species offer the most obstinate resistance to all at- 

 tempts. In the vegetable kingdom, Liliaceae, Rosacese, Salicaceae ; 

 and in the animal kingdom, Trout, Carp, Finches, etc., readily 

 produce hybrids. Many dogs, too, which differ considerably in 

 bodily structure, such as the dachshund and the pointer, the 

 retriever and the St. Bernard, produce mongrels. 



Further we see how unaccountable are the factors which are 

 dealt with in hybridisation when we consider the following 

 phenomenon : very frequently the ova of species A may be fer- 

 tilised with the spermatozoa of species B ; whilst, on the other 

 hand, the ova of B cannot be fertilised with the spermatozoa of A. 

 Thus sexual affinity between the sexual cells of two species is 

 present in the one case and absent in the other. It seems to me 

 that the determining factor should be sought for in the organisation 

 of the ovum, as may be concluded from the experiments cited 

 below. 



A few examples of one-sided crossing may be quoted. The ova 

 of Fucus vesiculosus may be fertilised with the antherozoids of 

 Fucus serratus, but the reverse cannot occur. Mirabilis Jalapa 

 produces seed when fertilised with the pollen of Mirabilis longi- 

 flora, whilst the latter remains unfruitful, if the attempt be made 

 to fertilise it with pollen from the former. 



Similar cases often occur in the animal kingdom, and amongst 

 these the most interesting are met with in those species in which 

 fertilisation can be induced artificially by mixing the sexual pro- 

 ducts. 



