more or less segregated in the reproductive centres it throws off" (p. 283). Here again it may be asked, by what process of crystallization thousands and tens of thousands of heterogeneous physiological units come to be segregated in the germ? The force of crystallization, as known to physicists, would tend to separate not to segregate them.
"Crystals grow by the addition of regular layers of molecules, arranged just like all other layers. We can set no limit to the size of a crystal so long as the supply of materials and conditions favourable to its formation remain constant. There is in fact the widest divergence in the size of crystal individuals of the same composition and structure. Those of ultra-microscopic dimensions and those many feet in length may be identical in everything but size. Both are equally complete, and one is in no sense the embryo of the other. As a rule, the size of a crystal is inversely proportionate to its purity and perfection of form, but this, as will be seen at once, is dependent on external conditions.
"Finally, the individual crystal, unlike the individual organism, will remain unchanged so long as its surroundings are favourable to existence."—Crystallography, pp. 10–1.
But living organisms grow after an altogether different fashion, they do not grow by additions to their outer layers. Small crystals resemble large crystals of the same kind in all except size, but embryos do not so resemble adults of the same species; they resemble more closely lower animals. The changes undergone by the embryo, explicable by the cell theory, but inexplicable by Mr. Spencer's theory, are alone sufficient to prove that the development of the organism is not a process of crystallization.
Mr. Spencer says, "While an aggregate of physiological units continues to grow,... no equilibrium can
