THE DEVELOPMENT OF ORGANISMS 799 



been wrought out in the course of time by Nature's automatic 

 sifting. 



{b) Something must also be allowed for size. Thus Prof. Sedgwick, 

 a very shrewd zoologist, pointed out that "the duration of gestation 

 depends on the size of the body and on the stage of development 

 at which the young are born". This statement is perhaps a little 

 tautologous in its second statement, since it is, in many cases, the 

 length of gestation that seems to make the difference between being 

 born rather helpless and being born with precocious powers. But 

 as to the first statement, there is no doubt that a large animal, 

 sach as an elephant or a whale, demands a long gestation. But 

 even this conclusion must not be pushed too far, for wild swine 

 have a gestation of about four months, about the same as in the 

 much larger lion. 



Thus we wish to rehabilitate the suggestion of Robert Chambers, 

 that the lengthening out of the period of gestation is an adaptive 

 change which has had as its survival value the advantage that the 

 offspring are born with better brains and otherwise more fully 

 equipped for the struggle for existence. 



DURATION OF LIFE 



Most animals, clearly, have a normal specific size, to which 

 the great majority of the adult members of the species closely 

 approximate. In a large collection representing a species there 

 may be a few giants and a few dwarfs; but most of the members 

 show a close approximation to the same limit of growth: and 

 there are good reasons for believing that this normal specific size 

 is adaptive; i.e. that it has been slowly established in the course of 

 selection as the fittest size for the given organisation and the given 

 conditions of life. In some cases, e.g. many fishes, there is no such 

 definite limit of growth; thus haddocks may be found as large as 

 average cods, e.g. over two feet long. 



Similarly in many animals that have been carefully studied we 

 find that there is a normal potential duration of life, — an age which 

 is rarely exceeded, though it may be seldom attained. This normal 

 "lease of life" is in most cases known only in a general way; but in 

 many cases we are able to say that the living creature in question 

 never lives longer than a few months, or a year, or a few years. 

 Statistics from forms kept in captivity are obviously vitiated by the 

 artificial conditions, and the life of animals in their natural condi- 

 tions is so often ended by "a violent death" — coming sooner or later 

 according to the varying intensity of the struggle for existence — . 

 that it is difficult to say what the normal potential duration of life 

 really is. But a critical survey of a large body of facts led Weismann, 



