48 THE EVOLUTION THEORY 



male to the female individual, and conversely, by virtue of the 

 continuity of the germ-plasm, so that the idea that sex can have 

 anything to do with the relative strength of the ids is altogether 

 erroneous. 



But, as I have already said, unilateral inheritance occurs even 

 in the mingling of species-characters, and most clearly in the case 

 of plant-hybrids. Thus, for instance, hybrids between the two species 

 of pink, Dianthus barbatus and Dianthus deltoides, resemble the 

 latter species much more closely than the former, and the hybrid 

 between the two species of foxglove which are wild in Germany, 

 Digitalis purpurea and Digitalis lutea, is much more like the latter 

 than like the former. 



It might be reasonably asked whether, in these crossings, the 

 normal number of ids in one species is not greater than in the other. 

 We know that, among animals at least, differences in the normal 

 number of chromosomes occur even in very nearly related species. 

 It is not impossible that this, in many cases, is really the cause of 

 the diversity of transmitting power in different species. Neverthe- 

 less, we cannot rest satisfied with this, for, in the first place, 

 this cause could not apply to the apparent unilateral inheritance 

 from one parent in Man, since the normal number of ids, as far 

 as we know, is strictly maintained in the same species, and second, 

 this would not explain certain phenomena of inheritance in plant- 

 hybrids. 



It happens not only frequently, but usually, that the different parts 

 of the hybrid take after one or other parent in different degree, and 

 this is the case also with children. In the hybrid between the two 

 species of tobacco-plant, Nicotianu rustica and i\^. pianiculata, which 

 I have already given as an example of a medium form between 

 the two parents, such diversities occur i-egularly in all the hybrid 

 individuals. Thus the corolla-tube of the hybrid is nearer N. 2xmicu- 

 lata in regard to its length, but nearer JV. rustica in regard to its 

 breadth. Many hybrids suggest one parent-form in the leaves, 

 the other in the blossoms. In the same way in the child the form 

 of ej'e may be that of the father, the colour of the iris that of 

 the mother, the nose maternal, the mouth paternal — in short, the 

 preponderance in heredity swings hither and thither from part to 

 part, from organ to organ, from character to character, and this 

 is even the rule though the oscillations may not always be apparent 

 and are often invisible. 



If we think of the proposition we arrived at eai'lier, and which 

 was proved chiefly by the case of identical twins, that the facies 



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