68 ORGAXIC EVOLUTION — THE FACTORS 



more or less segregated in the rei^roductive centres it 

 throws ofi"" (p. 288). 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, Avould 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 cr3\stal 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 per- 

 fection of form, but this, as will be seen at once, is 

 dependent on external conditions. 



" Finall}^ the individual crystal, unlike the individual 

 organism, wall remain unchanged so long as its sur- 

 roundings are favourable to existence." — Crystallogra'phy, 

 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 embiyos do not so 

 resemble adults of the same species ; they resemble more 

 closely lower animals. The changes undergone b}^ 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 physio- 

 logical units continues to grow, ... no equilibrium can 



