156 CAUSES OF VARIATION 



organ we do not know. Food and climate undoubtedly exert a 

 general influence, as we shall see, but altogether aside from this 

 there must be profound internal forces or interrelationships, 

 upon the normal exercise of which all typical results depend. 



Consider the development of a normal individual from the 

 fertilized ovum to maturity. The circumstances require not only 

 that arm, leg, and bone, heart, liver, and brain, arise at the proper 

 time and place, but also that the attendant cell divisions in each 

 proceed to the requisite number and then stop. If the number be 

 too few, a dwarf is the result; if too large, a giant ; and if too 

 few in some parts (arrested development) or too large in others 

 (hypertrophy), the individual is thrown out of proportion and is 

 recognized as more or less of a monstrosity according to the 

 degree of disproportion. To be sure, all these things occasion- 

 ally happen, and yet, in the majority of cases, the process of 

 cell division is adjusted with a nicety that is nothing short of 

 marvelous ; in any event, the results secured, though varying 

 somewhat in total development, are yet almost absolutely 

 proportional (P). 1 



Whatever may be the controlling force to decide at what point 

 cell division in each case shall stop and when the individual as 

 a whole shall cease to grow, the plain physiological fact is that 

 all considerations of size (quantitative variation) are fundamen- 

 tally those of cell division. 



The cessation of growth at maturity does not imply the loss 

 of power of cell division, because most forms of life, plant or ani- 

 mal, have more or less powers of regeneration if a part is lost 

 or injured. If a leg of a salamander be cut away, it will speedily 

 be restored, bones and all, as good as new. A tail of a lizard is 

 readily broken off, separating not between two vertebrae but at 

 the middle of a vertebra (in some species generally the seventh 

 caudal). 2 When the tail regenerates, however, the vertebrae do 



1 At this point the author questions his own statement. As a matter of fact, 

 the data involved have not been submitted to absolute mathematical determina- 

 tion. We do not know whether the normal deviation in size due to variation in 

 cell division is the same for all species ; nor do we know whether in giants and 

 dwarfs all parts bear the same relative proportions as in normal specimens ; 

 indeed, there is ground for believing that they do not. In the most general 

 sense, however, the statement is true. 2 Morgan, Regeneration, p. 198. 



