Organic Evolution. 229 



a perfect queen. Not only are the sexual organs stimulated 

 to increased growth and functional activity, but the corre- 

 lated size and condition of the sense-organs are likewise 

 acquired. The characters of queen and worker are latent 

 in the grub. According to the nature of the food it receives, 

 the one set of characters or the other emerges. Professor 

 Yung's tadpoles and Mrs. Treat's butterflies (ante, p. 59) 

 afford similar instances. 



We come now to those cases of latency in which this 

 obvious correlation does not occur. They afford examples 

 of reversion to more or less remote ancestral characters. 

 In some cases the cause of such reversion — such unexpected 

 emergence of characters, which have remained latent 

 through several, perhaps many, generations — is quite un- 

 known. In others, at any rate among domesticated 

 animals, the determining condition of such reversion is the 

 crossing of distinct breeds. 



Darwin gives* an instance of reversion, on the authority 

 of Mr. E. Walker. He bought a black bull, the son of a 

 black cow with white legs, white belly, and part of the tail 

 white ; and in 1870 a calf, the gr-gr-gr-gr-grandchild of 

 this cow, was born, coloured in the same very peculiar 

 manner, all the intermediate offspring having been black. 

 In man partial reversions are not infrequent. An addi- 

 tional pair of lumbar ribs is sometimes developed, and in 

 such cases the fan-shaped tendons which are normally 

 connected with the transverse processes of the vertebras are 

 replaced by functional levator muscles. Since it is probable 

 that the ancestor of man had more than the twelve pairs 

 of ribs that are normally present in the human species, 

 we may, perhaps, fairly regard the supernumerary rib as a 

 reversion. But it may be a new sport on old lines. 



The occasional occurrence in Scotland of red grouse 

 with a large amount of white in the winter plumage, 

 especially on the under parts, is justly regarded by Mr. 

 Wallace f as a good example of reversion or latency in 



* " Animals and Plants under Domestication," vol. ii. p. 8. 

 f " Darwinism," p. 107. 



