106 THE CAUSATION OF DISEASE. 



of the developmental processes that further development shall 

 proceed on the old lines xa, xb. 



It may be that stripes like those met with in the zebra are 



Fig. 9. 



B 



D 

 Diagram showing how a character existing potentially in the embryo 

 may be converted into an actuality. 



always normally present in the embryo foal at some period of 

 its development, and, if such is the case, one may account for 

 their presence in the young foal by supposing a simple arrest 

 in development, be the cause what it may, peculiar E or not. 

 But let it be well noted that many reverted characters are 

 never actually present in the embryo — only latently present ; 

 and whether the zebra stripes are always present in the embryo 

 foal, or whether their occurrence in the young foal is merely 

 the conversion into an actuality of a power latent in the embryo, 

 matters not much. The diagram assumes the undoubted fact 

 that the embryo has latent powers which may or may not 

 develop into actualities, and we shall not err greatly if we 

 assume the pronounced stripes of the zebra to be only latently 

 present in the embryo foal. 



Whether this method of viewing the conversion of a latency 

 into an actuality be correct, I know not. It will be observed 

 that in this attempted explanation I am regarding the life of an 

 organism as a cycle — a complex rhythm, and the development 

 of a latency as the substitution of a previous rhythm for a present 

 one, owing to peculiarity of E. But, be the explanation what 

 it may, let us ever bear in mind that an organism is the seat 

 of many thousand such latencies. 



2. We now come to the second cause of reversion, namely, 



