THE CAUSATION OF DISEASE. 107 



the union of parents showing structural characters of diver- 

 gent character; in consequence of this divergence they refuse 

 to blend, and thus display a status quo ante. Such characters 

 are of more or less recent acquisition. 



It is a well-established biological fact that reversions most 

 frequently occur when distinct species, or pronounced varieties 

 of the same species, are crossed. Whatever the explanation, 

 the fact remains, and, as w r e shall see, it has a most important 

 application in medicine. In the fifth chapter of his " Origin of 

 Species," Darwin illustrates this result of crossing. He tells us 

 that zebra-like stripes frequently result from the crossing of 

 the several species of the horse genus : the ass, horse, quagga, 

 hemionus — all of which are descended from the zebra. Another 

 striking illustration of the same principle is afforded by the 

 pigeon. There are over three hundred varieties of this bird, all 

 of which are undoubtedly descended from the Blue Rock pigeon, 

 a bird very like our Wood-pigeon. Now, w T hen distinct breeds of 

 various colour are crossed, there is " a strong tendency for the 

 blue tail and bars and marks to reappear in the mongrels." 

 The author goes on to say that there is evidently c; a tendency 

 in the young of each successive generation to produce the long- 

 lost characters " (note this in reference to my remarks on 

 potentiality), c: and this tendency from unknown causes pre- 

 vails " in such crosses. The above hypothesis affords, I imagine, 

 some explanation. We may suppose, namely, that the recently 

 acquired characters refuse to blend ; that the parents are, in 

 fact, barren in respect of them, so that there is a reversion to 

 a status quo ante — to the condition of things which obtained 

 before the acquisition of the non-blending characters. 



Up to the point of ancestral divergence, any two different 

 species of the same genus may, in fact, be regarded as of the same 

 species, and therefore, as we should expect, the development of 

 the mongrel offspring thus far proceeds harmoniously ; for are 

 not these the lines on which it has proceeded for many genera- 

 tions — thousands, it may even be tens of thousands ? The germ 

 and sperm, thus far, are, so to speak, perfectly adapted to 

 each other; but beyond this point each element has dif- 

 ferent tendencies : from the one parent comes a tendency 

 to reproduce one set of peculiarities, and from the other 



