108 THE CAUSATION OF DISEASE. 



a tendency to produce another set — perhaps of a totally 

 opposite order. Therefore, henceforward the process of de- 

 velopment tends to be somewhat chaotic. The two tendencies, 

 coming into union for the first time, show a disinclination to 

 blend, and in this chaotic state, pre-potency no doubt plays an 

 important part, certain recently acquired characters occasionally 

 showing great proneness to appear in the offspring ; but the 

 general rule is that such recently acquired characters tend to be 

 dropped, the offspring revealing characters peculiar to a remote 

 ancestor — that is to say, the status quo ante the divergence. 

 Or, putting the matter in another way, we may say that the 

 two sets of characters acquired after the point of divergence 

 are barren, one with another. Viewed from the embryological 

 standpoint, each organism is, as far as this point, practically of 

 the same species, and therefore capable of perfect sexual repro- 

 duction one with the other, but after it the two are of different 

 species, and more or less barren. 



The above explanation may be rendered clearer by the 

 accompanying diagram. The line axb represents the develop- 



Fig. IO. 



B 

 / 



X 



ment of the integuments of the ass, axd that of the horse, 

 AC, AC = development of zebra. X in each line marks the 

 point of divergence. From A to x the lines are parallel. This 

 signifies that the developmental processes are thus far parallel 

 and in harmony. But beyond this point the diverging lines, 

 xb, xd, signify diverging tendencies. So that if axb represents 

 the germinal tendencies of the ass, and axd those of the horse, 

 there will be perfect union up to x ; beyond this xb, xc will 

 more or less refuse to blend, and development will tend to pro- 

 ceed on the old lines to C 



