CHAP. II.] INDIVIDUAL DIFFERENCES. 31 



define ; but here community of descent is almost universally im- 

 plied, though it can rarely be proved. We have also what are 

 called monstrosities ; but they graduate into varieties. By a mon- 

 strosity I presume is meant some considerable deviation of struc- 

 ture, generally injurious, or not useful to the species. Some authors 

 use the term " variation " in a technical sense, as implying a modi- 

 fication directly due to the physical conditions of life ; and " varia- 

 tions " in this sense are supposed not to be inherited ; but who can 

 say that the dwarfed condition of shells in the brackish waters of 

 the Baltic, or dwarfed plants on Alpine summits, or the thicker fur 

 of an animal from far northwards, would not in some cases be in- 

 herited for at least a few generations 1 and in this case I presume 

 that the form would be called a variety. 



It may be doubted whether sudden and considerable deviations 

 of structure such as we occasionally see in our domestic produc- 

 tions, more especially with plants, are ever permanently propagated 

 in a state of nature. Almost every part of every organic being is 

 so beautifully related to its complex conditions of life that it seems 

 as improbable that any part should have been suddenly produced 

 perfect, as that a complex machine should have been invented by 

 man in a perfect state. Under domestication monstrosities some- 

 times occur which resemble normal structures in widely different 

 animals. Thus pigs have occasionally been born with a sort of 

 proboscis, and if any wild species of the same genus had naturally 

 possessed a proboscis, it might have been argued that this had 

 appeared as a monstrosity ; but I have as yet failed to find, after 

 diligent search, cases of monstrosities resembling normal structures 

 in nearly allied forms, and these alone bear on the question. If 

 monstrous forms of this kind ever do appear in a state of nature 

 and are capable of reproduction (which is not always the case), as 

 they occur rarely and singly, their preservation would depend on 

 unusually favourable circumstances. They would, also, during the 

 first and succeeding generations cross with the ordinary form, and 

 thus their abnormal character would almost inevitably be lost. 

 But I shall have to return in a future chapter to the preservation 

 and perpetuation of single or occasional variations. 



Individual Differences. 



The many slight differences which appear in the offspring from 

 the same parents, or which it may be presumed have thus arisen, 

 from being observed in the individuals of the same species inhabit- 

 ing the same confined locality, may be called individual differences. 

 No one supposes that all the individuals of the same species are 

 cast in the same actual mould. These individual differences are of 

 the highest importance for us, for they are often inherited, as must 

 be familiar to every one ; and they thus afford materials for natural 



