STABILITY OF PARTHENOGENETIC TYPES 201 



jugation. In every instance the result is a blend which persists 

 in subsequent generations, not a Mendelian mixture. Here, 

 very clearly, the effect of blending is retrogression towards the 

 parental and varietal mean. If many races interbred it would 

 imply retrogression, within the limits of their differences, towards 

 the specific mean, consequently towards the ancestral type. Thus, 

 if races which have undergone evolution against different diseases 

 cross, the resisting powers of those races which have undergone 

 progression are diminished in the descendants. 1 



329. We have used the term blending as including all degrees 

 of prepotency up to the exclusive inheritance of the character of 

 the one parent and the permanent loss of that of the other. Old 

 established characters tend always to be prepotent over newer 

 characters, as is proved by the fact that when parents differ the 

 progressive variations of one generation tend not to be reproduced 

 by the next. It is also proved by another set of facts, namely, 

 that retrogression of even old-established parts is a constant ac- 

 companiment of cessation of selection. We have no reason to 

 doubt that, apart from the effects of blending, offspring tend to 

 vary from their parents retrogressively as well as progressively. 

 Evidently these retrogressions which imply reversions to an old 

 type, tend to be prepotent over the type which has become the 

 normal of the race ; hence, in part at least, retrogression in the 

 absence of selection. 



330. The little evidence we possess concerning parthenoge- 

 netic species points in the same direction. Their useless characters 

 (e.g. receptacula seminis in Cypris reptans) are very persistent. 

 Parthenogenetic types are exceptionally prolific of progressive 

 varieties, a fact which clearly indicates comparative lack of regres- 

 sion and, therefore, of retrogression. 



331. The fact that under conditions of parthenogenesis (complete 

 or partial) " thousands of forms may be cultivated side by side 

 . . . and exhibit slight but undoubted differentiating features, and 

 reproduce themselves truly by seed " 2 is very significant. It affords 

 evidence (i) that progressive variations are then plentiful, and 

 therefore that bi-parental reproduction is not the cause of them ; 

 (2) that such variations are then especially persistent, and therefore 

 that conjugation tends to plane them away, and (3) that fluctuations 

 are unstable, not because they are due to " varying conditions of 

 food supply, temperature, density, moisture, light," etc., but because 

 they blend with one another in bi-parental reproduction. Otherwise, 



1 See 464. 2 See 244 (footnote). 



