398 SCIENCE PROGRESS 



the plausible suggestion that the mutations arose in the first 

 instance by some accident or irregularity in nuclear division, 

 so that one of the two daughter nuclei which were produced 

 received less than its share of the chromatin. When once this 

 has occurred, if such a defective germ-cell unites with a perfect 

 one, the peculiar nuclear divisions which usher in the ripening 

 of the germ-cells in the resulting organism will account for the 

 production of two types of germ-cell. 



When we read over the list of " mutations " which have 

 cropped up in the cultures of Mendelians, we are struck with 

 the fact that practically all represent deficiencies and patho- 

 logical aberrations totally unlike the marks which separate 

 natural species from one another. Dr. Bateson, in his presi- 

 dential address to the British Association in 19 14, even tenta- 

 tively suggested that evolution might have consisted in the 

 development of a series of defects, and that all the qualities 

 of Shakespeare were implicit in the original amoeba. He was 

 far too good a naturalist to suggest that such a theory should be 

 adopted ; he only indicated that such a possibility should be 

 considered. When we contemplate, however, such mutants 

 as white-eyed flies, flies with wings the layers of which are 

 separated by bubbles of air, flies with shrivelled wings, we 

 come to the conclusion that such deviations from the normal 

 have played no part in evolution. 



If such mutations, then, have not formed the raw material for 

 evolution, can this have consisted in the trifling differences in 

 size, form, and colour which, as Darwin pointed out, are to be 

 found amongst the members of every family, brood, and litter, 

 differences which, he assumed, could be indefinitely increased 

 by selection continued through many generations ? To this 

 theory the pure-line investigations have given the death-blow. 

 As an example of these we may take the investigations of Prof. 

 Agar on the heredity of the water-flea Simocephalus. 



This is a little Crustacean provided with a bivalve carapace. 

 The space between the body and the carapace serves as a brood- 

 pouch, and into it the eggs are laid, and they are carried about 

 by the mother until they are ready to take up an independent 

 existence for themselves. These eggs develop parthenogenetic- 

 ally, that is without being fertilised by a male. Now, if we 

 select a single female and isolate her, and rear her progeny to 

 maturity, when in turn they will produce eggs, we establish 

 what is termed a " pure line." Amongst the brood there will 

 be slight variations in the length of the carapace. If we en- 

 deavour, however, to get a strain with longer carapaces by 

 selecting the longest individuals for further propagation, we fail ; 

 and the same fate befalls us if we try to raise a strain with shorter 

 carapaces. Agar found, in fact, that the average carapace length 



