A PHYSIOLOGICAL CHARACTER. 139 



that the minus strain of Line 757 through the selection was very 

 little, though somewhat improved; (5) that this improvement was 

 permanent, or at least persisted for 32 months after the close of 

 selection. 



The writer challenges the first of these assumptions as contrary 

 to fact. 1 Further, at no time was there any observable difference 

 in the general activity of the two strains of Line 757, except when 

 subjected to directive light stimulation in the experimental tank. 

 To the second, third, and fourth assumptions the writer makes no 

 objection, except the general objection that there is no obvious ad- 

 vantage derived through them and they seem uncalled for; further, 

 there is no obvious reason for assuming an obscure physiological 

 effect rather than genetic changes. The last of these assumptions 

 appears tantamount to the assumption of a permanent extra-chro- 

 mosomal inheritance or at least an extra-genetic transmission — an 

 extremely precarious assumption. 



It is possible to assume that the increase in reactiveness as the 

 experiment progressed was due to a better handling of the material, 

 such as to affect the reactiveness (but not the reproductive index), but 

 the author was not conscious of any better manipulation. 



The logical explanation would seem to be that the pond-water 

 which was used in testing these animals was undergoing a progressive 

 change. This pond is fed by a spring and has considerable overflow. 

 It is a dying pond, rapidly filling with silt. At the close of the experi- 

 ment the spring-water was flowing through a pond which then had 

 probably less than one-third the water-content which it had at the 

 beginning of the experiment. The water was always taken at the 

 outlet. Consequently, as the experiment progressed, the water used 

 in the experimental tank had remained for shorter and shorter periods 

 of time in the pond from which it was obtained. It is reasonable to 

 suppose that this water was progressively changed in some of its 

 constituents (carbon dioxide, oxygen, substances obtained from the 

 silt, or in other constituents). This explanation also seems plausible 

 in view of the fact that reaction-time is clearly closely related to the 

 condition of the water in which the young daphnids are tested. 2 



Whatever may have been the cause for the generally reduced 

 reaction-time for S. exspinosus strains during the latter part of the 

 experiment, it should be borne in mind that except for the practice 

 of selection all strains were treated as nearly alike as possible and 

 that there is apparently no ground for the assumption of a possible 

 continued differential effect due to selection other than such as came 

 about through genetic changes. 



1 The validity of the reproductive index as a measure of vigor (including general muscular 

 activity) is discussed on page 143. 



2 Local influences, presumably due to changes in the water, have been frequently referred to 

 (see table 2, pages 17-1S, and elsewhere). 



