EVOLUTION OP THE COLORS OF BIRDS. 55 



freely confesses his ignorance on this point, saying: 

 " Our ignorance of the laws of variation is profound. 

 Not in one case out of a hundred can we pretend to as- 

 sign any reason why this or that part has varied."* 

 Spencer suggests a number of causes for variation. f He 

 considers the influences of environment in altering 

 functions to be one factor in determining variations, 

 asserting * * * " that organisms produced by the 

 same parents at the same time, must be more or less 

 differentiated both by insensible initial differences, and 

 by slight differences in the conditions to which they are 

 subject during their evolution." He then appeals to first 

 principles to show that no two parts of a homogeneous 

 substance can be exactly alike, and that consequently 

 there must always be a difference at least in the number 

 of physiological units composing a reproductive cell. 

 Thus we have a clue to the differences existing in the 

 young of a single litter. 



Weismann's theory of variation is probably the most 

 carefully worked out in its details, and is perfectly con- 

 sistent with his theory of the isolation of the germ plasm. 

 It appears impossible, however, that any progressive 

 modification could take place according to this view, for 

 it depends exclusively upon the number of ahnenplas- 

 mas of each sex which enter into combination, this 

 number being largely determined by the amount of sur- 

 plus germ plasm which is disposed of in the extrusion 

 of the two polar cells. But variation according to this 

 theory, it would seem, must be strictly conservative, for 

 all modifications must be within the extremes of an- 

 cestral modification. Thus, let us suppose that every 

 living individual of some species has been measured, 

 and that 500 represents the number of units in the 



* Origin of Species, p. 73, Humboldt. 

 t Principles of Biology, I, pp. 257-272. 



