EVOLUTION OF THE COLORS OF BIRDS. 71 



another black form, the jack-rabbit of Espiritu Santo 

 Ishmd, Lower California, Lepus insularis, in which both 

 male and female are equally black. This law, as stated 

 in Zoe, referred only to transformations of color, but 

 would apply equally to any form of modification. Varia- 

 tions, according to the principles stated in this chapter, 

 are due to influences of the outer world upon the organ- 

 ism. One of the most potent environmental influences 

 is nutriment. If there be an abundance of nourishment 

 during a period of time, the tendency wdll at first be 

 anabolic, or constructive. There is, at such times, a 

 storing away of energy for future use. Geddes and 

 Thomson have shown that a preponderance of females 

 is born in times of plenty, of males in times of want. 

 After a time of bountiful nourishment suppose a period 

 of scanty food supply to follow. There will be a ten- 

 dency toward katabolism, which means a breaking up of 

 the energy which has been accumulating, a tendency 

 toward variation and a preponderance of males. The 

 males being more katabolic than the females, will tend 

 to vary in advance of the latter in every direction in 

 which a surplus of energy might be expended. In 

 noting the different kinds of variation which have been 

 observed it was found that they might consist of differ- 

 ences in size, shape, position, number, color or the 

 addition of parts. It is obvious that any increase of 

 size of the whole organism or of any particular part is 

 due to an increase in the number of constituent cells, 

 while variations in shape are due simply to a different 

 distribution of the cells or to an increase or decrease in 

 the number in any particular region. Variations such 

 as ordinarily occur in organisms then, are apparently 

 either variations in number or position. Number and 

 position depend on the amount and location of growth- 

 force, which in turn is conditioned by the metabolism 



