EVOLUTION 983 



characters, it was often held to be true as regards the race. But a 

 strong reaction has set in. Let us briefly consider why. 



(i) If we take the eggs of, say, the blackheaded gull, and hatch 

 them in an incubator in the laboratory, and rear the results in 

 confinement, we get, as everyone knows, a number of normal, well- 

 endowed young birds, which will migrate months afterwards when 

 their kinsfolk fly about overhead. The environment, the whole 

 nurture, was in this case very peculiar, but it did not seem to make 

 much difference. There is evidence, indeed, that birds which have 

 not known freedom are badly handicapped when liberated, because 

 they do not know their way about. But the clear fact seems to be 

 that for many creatures changes of nurture do not fundamentally 

 matter as long as the essential conditions of life are not interfered 

 with. The full inheritance may not be expressed, but a large propor- 

 tion of it is realised as usual. There are many delicate creatures, 

 such as the larvse of sea-urchins, which are difficult to rear, which 

 do not readily stand even slight nurtural changes; but many other 

 creatures can within limits adjust themselves to, and develop 

 normally in, quite peculiar conditions of life. It is a very striking fact 

 that the ovum of a rabbit can develop for two days normally outside 

 of the body; and Man is peculiarly master of his Fate. What, then, 

 is the importance of nurture ? 



(2) It is likewise a familiar fact that there is often an extra- 

 ordinary tenacity in the persistence of hereditary characters, no 

 matter how the nurture is changed. Having all the fingers thumbs 

 has been known to persist for six generations, night-blindness in a 

 lineage for two and a half centuries, a particular kind of dwarf for 

 four generations. A peculiar variety of the Greater Celandine with 

 cut-up leaves which appeared suddenly in an apothecary's garden 

 at the end of the sixteenth century has bred true ever since. "He 

 that will to Cupar maun to Cupar", we sadly say in Scotland. 

 "Each man's nature is his fate", said Democritus, and the modem 

 students of heredity agree. What, then, is the importance of 

 nurture ? 



(3) Another consideration is this. It is not difficult to impose 

 peculiarities on organisms by subjecting them to peculiarities of 

 nurture. A goldfish kept continuously in the dark becomes quite 

 blind; caterpillars subjected to cold may turn into dark-coloured 

 butterflies ; some birds, such as the bobolink, may be dieted so that 

 they keep their breeding plumage through the year and will sing 

 their spring song in mid-winter. These are three instances out of 

 hundreds of the power of nurture, to which we shall return. But, 

 as things stand, we cannot assert that any one of these extrinsic 

 modifications is as such, or in any representative degree, entailed 

 on the next generation. So what is the importance of nurture? 



(4) We have the statistical evidence furnished by Karl Pearson 



