THE CAUSATION OF DISEASE. 105 



vidual has shown marked likeness in his ways and instincts 

 to certain of the lower animals. There can be little doubt, I 

 imagine, that the arrest in such cases is due to peculiarity of 

 ante-partem E. As an example of reversion, due to peculiar 

 ante-partem E, causing not a simple arrest merely, but such a per- 

 version of embryonic processes, that development proceeds on 

 old lines, we may instance the stripes which sometimes occur 

 on the young foal. These are evidently derived from the 

 zebra — a far-off progenitor of the horse. The fact that they 

 are observed every now and again in the foal shows that they 

 must always potentially exist in the embryo. What is it 

 that causes this potentiality every now and again to become 

 an actuality ? The union of parents having unconibinable 

 tendencies may afford the true explanation of the reversion 

 — of which more presently. Another possible cause of 

 the reversion in question is the one under consideration, 

 namely, a peculiarity in antc-parlcm E, which prevents the 

 development of the skin from proceeding on the lines peculiar 

 to the horse, causing it, on the other hand, to progress on more 

 ancient lines. Thus, at the time of foetal life, when the 

 embryo is most like the zebra as to its integuments and such 

 tissues as influence their development, one may suppose some 

 such accidental dislocation of the molecular processes of the 

 one or the other to take place, that the further development 

 of the integuments proceeds on the old lines peculiar to the 

 zebra. 



This mode of viewing the matter may be rendered clearer 

 by a diagram. In Fig. 9, ab represents the fully developed 

 zebra skin ; CD, the fully developed horse skin ; o being the 

 starting point of development. Up to the point x we may 

 suppose the development the same for each. But hencefor- 

 ward there is a divergence, — in the case of the zebra develop- 

 ment proceeding in the direction of A and B ; in the case of the 

 horse, of C and D. So far as the latter is concerned, the un- 

 shaded part represents a dropped character (this being dia- 

 grammatically indicated as in Fig. 8). 



Now, if we suppose that the integuments of the foal have 

 arrived in their development at the point x, we can imagine 

 that a peculiarity of E might lead to such molecular dislocation 



