154 SELECTION IN CLADOCERA ON THE BASIS OF 



Cladocera material has the advantages for a selection experiment 

 possessed by protozoa in that it is (under controlled conditions) 

 exclusively uniparental in inheritance. It has additional advantages 

 in that the germ-plasm is less bound up with the soma than in 

 protozoa and the very great advantage that (so far as our knowledge 

 goes, at any rate) there are no complications of chromidia or separate 

 nuclear masses within the germ-cell, recombination of which might 

 conceivably produce the results attained by selection in protozoa. 



So far as known, there is no mechanism within the germ-cells 

 of Cladocera through which a segregation or recombination of nuclear 

 material is to be expected in parthenogenesis. In the production 

 of the parthenogenetic egg there is a single maturation division 

 (Weismann, 1886; Kiihn, 1908), in which the chromosomes behave as 

 in any ordinary cell-division, i. e., each chromosome is merely divided 

 into two theoretically equal parts, equal quantitatively and quali- 

 tatively. Hence the objections offered to results of selection with 

 protozoa do not apply to the present material. Any results obtained 

 from selection in such material (parthenogenetic and without reduc- 

 tion) should have a crucial bearing on the problem of selection. 



In experiments having to do with the variability of organisms 

 it does not seem necessary to suppose that all apparently similar 

 material should be found similarly subject to genetic variation. 

 In Stocking's (1915) Paramecium, about 20 per cent of the strains 

 in which an abnormality was hereditary showed genetic variation. 

 In the Cladocera material used in selection, Line 757 alone has 

 certainly shown such genetic variation with regard to the character- 

 istic studied, though the possible (but doubtful) mutation in the 

 minus strain of Line 740 and the somewhat consistent differences 

 in mean reaction- times between the two strains of Lines 689, 691, 

 711, 719, 762, 794, and 795 are suggestive of differences of genetic 

 significance. Allowing that the case is proven for Line 757 alone 

 (though some of the other cases are very suggestive), genetic variation 

 was found in only one of the 15 distinct lines of Cladocera studied. 

 This is approximately only 7 per cent. 



The effects of selection in Line 757 may be assumed to be 

 either (1) of a nature of general physiological changes, possibly 

 changes in metabolism or some kindred effect, or (2) direct genetic 

 changes. 1 



If the former interpretation is favored, the question arises as 

 to what is the basis of such physiological effects. If they are con- 

 sidered not strictly genetic, the apparently permanent nature of the 

 change is a matter of especial importance. Cases of transmission 



1 In this discussion genetic variations, in metazoa at least, are assumed to concern genetic 

 factors confined to the chromosomes. The carrying over from one generation to another of 

 cytoplasmic inclusions and the transmission of variations or modifications which affect only one 

 or two generations are assumed to concern the cytoplasm alone and are considered not strictly 

 genetic. 



