The Making of Species 
lateral triangle, six of them going to each side. 
Suppose now, that from some cause or other they 
rearrange themselves to form an isosceles triangle, 
so that only four form the base and seven go to 
each of the remaining sides. Such an arrange- 
ment would give rise to a mutation. Suppose now 
that, from some cause or other, this triangular bio- 
logical molecule were to split up into two triangles, 
each having three atoms to each side, we should 
obtain a still more marked mutation. We are 
far from saying that the atoms in the organic 
molecule ever take such forms. We have merely 
attempted to give rough but simple illustrations 
of the kind of processes which on this hypothesis 
might be expected to take place in the germ cells 
or the fertilised eggs. 
Let us now consider the sexual act from this 
aspect. The various molecules (we speak, of 
course, of biological molecules) of the male 
parent meet those of the female parent, and a 
synthesis occurs, which results in the formation 
of a new organism. When these two sets of 
gametes meet one another, one of several events 
may happen. The gametes may refuse to com- 
bine. This will occur whenever they are of 
very different constitution ; thus it is that widely 
differing species will not interbreed. But it may 
even happen that gametes of individuals of the 
same species may refuse to coalesce on account 
of some peculiarity in the composition of one or 
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