40 THE GROWTH OF THE BKAIX. 



thus bringing to light a mechanism for symmetrical 

 growth. 1 



The relations between geographical distribution and 

 the colour and size of various birds and mammals of 

 North America have been studied by Baird, Allen, and 

 others. 2 They found that the largest individuals were 

 obtained from localities at which the type was most 

 numerously represented. Such localities are designated 

 as the centres of distribution, and are assumed to be 

 those in which that type finds the best conditions for 

 existence. Many mammals and birds are found to have 

 their centres of distribution in the northern regions, and 

 so to diminish in size from the northern to the southern 

 latitudes, thus showing that mere increase in tempera- 

 ture, which at first sight might be expected to favour 

 size, does not do so in these cases. Reversed instances, 

 where the centres of distribution are in southern latitudes, 

 also occur, but they are not so numerous. Familiar are 

 the enormous possibilities of variations in size among 

 animals under domestication, and our small and large 

 dogs and horses are sufficient indication of what can be 

 done in part by selection. But it is quite in accordance 

 with these facts to look upon size as a character mainly 

 predetermined in the egg, and only to a moderate degree 

 influenced by conditions acting through the lifetime of 

 the individual. 



The increase in stature and weight of the higher 

 animals has been found to show a periodic variation in 

 the rate at which it goes on, whether the increase is 

 due to the multiplication of cells, or simply to their 

 enlargement. In many eggs when dividing there are 

 between the periods of actual division long intervals in 



1 Samuel, Virchow's Archiv., 1887. 



2 Allen, Radical Review, 1890. 



