THE VITAL PROPERTIES OF THE CELL 319 



differences by producing a new individual, which occupies a mean 

 position between its parents. By this means numberless new 

 varieties are developed, which only differ slightly from one 

 another. Hence Weismann (IX. 34) is of opinion that fertilisa- 

 tion is an arrangement by means of which an enormous number 

 of varying individual combinations arise ; these supply the 

 material for the operation of natural selection, the result being 

 that new varieties are produced. 



Whilst agreeing with the first part of this principle, I cannot 

 support the second. The individual differences which are called 

 into being by fertilisation, and which furnish the basis for 

 natural selection, are as a rule only of an insignificant nature, and 

 are always liable to become suppressed, weakened, or forced into 

 another direction, by some subsequent union. A new variety can 

 only be formed, if numerous members of a species vary in a 

 definite direction, so that a summation or strengthening of their 

 peculiarities is arrived at, whilst other individuals of the same 

 species, which preserve their original characters, or vary in another 

 direction, must be prevented from uniting sexually with them. 

 Such a process presupposes the presence of an environment which 

 always acts in a constant manner, and the existence of a certain 

 intervening space between the two sets of individuals belonging 

 to the species, which is destined to divide into two new species. 



Sexual reproduction, therefore, seems to me to influence the 

 formation of a species in a manner opposed to that suggested by 

 Weismann. By creating intermediate forms, it continually re- 

 conciles the differences which are produced by external circum- 

 stances in the individuals of a species ; thus it tends to make the 

 species homogeneous and to enable it to retain its own peculiar 

 features. Here, too, sexual affinity, that mysterious property of, 

 organic substance, by preventing a combination, or at any rate a I 

 successful one, between substances which are either too similar or 

 too dissimilar, acts as an important factor. For, if the sexual 

 products, on account of their different organisation and their 

 slight sexual affinity, cannot mingle successfully, the species and 

 orders in question are kept apart. 



Darwin and Spencer express the same opinion. According to 

 the former, "intercrossing plays a very important part in nature, in 

 keeping the individuals of the same species or of the variety true 

 and uniform in character." And Spencer remarks : " In a species 

 there is, through gamogenesis, a perpetual neutralization of those 



