150 EVOLUTION AND ANIMAL LIFE 



other words, that when compared with a normal five-segmented foot it 

 would appear to be a modification of such a foot with some one segment 

 wanting. But that condition is not at all what appears after the cock- 

 roach regenerates a foot. The new foot is only very little, if any, 

 shorter than the normal five-segmented foot (see Fig. 89) : one cannot 

 say that it is precisely this or that segment which is lost. It is a new 

 kind of foot, apparently just as capable, as 'fit/ as useful as the five- 

 segmented kind. We have regularly occurring, in these cases of re- 

 generation, the development of an entirely changed organ, similar as 

 a whole to the old one, but different from it in all its parts; this differ- 

 ence not being one of incompleteness, or serial addition or subtraction, 

 but the difference of newness. It is the regenerative mutation of an 

 organ!" 



In five years of experimental rearing of the silkworms for 

 the sake of studying phenomena of heredity and variation, the 

 junior author has been able to record numerous cases of discon- 

 tinuous or sport variation such as the absence of the usually 

 well-developed caudal horn of the larva, melanism in larvae, 

 double cocooning or absence of cocoon in the pupal condition, 

 congenital monstrous loss of a whole wing in the adult, striking 

 aberration of the wing pattern in the adult, etc. But the great 

 mass of variation ever present and readily observable among 

 the scores of thousands of silkworm individuals reared and 

 carefully scrutinized has been of the continuous (fluctuating or 

 Darwinian) type. 



A special type or kind of discontinuous variation, that 

 exemplified by the so-called de Vriesian mutations, is discussed 

 at the end of this chapter. 



The matter of determinate variation is discussed as follows 

 in the same paper : 



"The theory of determinate variation is based on the hypothesis 

 that fluctuating variations are not in all cases, nor necessarily in any 

 case, purely fortuitous and scattering, but that because of some in- 

 trinsic or extrinsic influence they tend to occur along definite or 

 determinate lines. The need for the theory rests on the claimed inad- 

 equacy of slight fortuitous variation in offering selection a sufficient 

 ' handle ' for action. The greatest logical difficulty with the theory is 

 that none of the influences which are known is adequate to cause such 

 an effect as that of producing persistent determinate variations. In 

 the case of any developing individual, determinate variation can be 



