i82 NATURAL SCIENCE. ^^,._ 



The great variability under cultivation Dr, Weismann refers to 

 " Panmixia," or the cessation of Natural Selection, with the conse- 

 quence of indiscriminate crossing. He thus describes it : — " As soon 

 as natural selection ceases to operate upon any character, structural 

 or functional, it begins to disappear " {Note, p. 140). If so, and if 

 Natural Selection is withdrawn under cultivation, thus ceasing to 

 keep up any (and that is equivalent to saying all the) characters of 

 plants to the required standard, then all the organs of a plant ought 

 to degenerate and atrophy. But hypertrophy and not atrophy is the 

 most obvious characteristic of all cultivated plants. 



Again, he says : — " As soon as an organ becomes useless, the 

 continued selection of individuals in which it is best developed must 

 cease, a.nd panmixia takes place" (p. 291). 



It is not easy to find cases of degeneracy in cultivated plants, 

 but the two following may suffice. In Bellevallia comosa the terminal 

 flowers of the raceme are already abortive in the wild state. Under 

 cultivation they all deteriorate and become rudimentary, or vanish 

 altogether, but when this stage is arrived at, the flower stalks, now 

 absolutely useless, become subject to hypertrophy, so that a large 

 feathery process of violet-coloured pedicels is obtained. Similarly in 

 the Guelder-rose, as soon as the central and normally perfect flowers 

 have lost their sexual organ$ under cultivation, the corollas, now 

 useless to attract insects for fertilising the pistils, become increased in 

 size, until a great globe of barren flowers is the result. According to 

 Dr. Weismann, both the flower-stalks of Bellevallia and the corollas of 

 Viburnum opulus should have atrophied and disappeared. Similarly 

 we ought not to have any " double " flowers at all, and garden roses 

 should be a thing unknown.'/ 



Lastly, by panmixia there " follows a mixture of all possible 

 degrees of perfection, which must in the course of time result in the 

 deterioration of the average development of the organ " (p. 292). 



In making this somewhat categorical statement, Dr. Weismann 

 does not appear to have allowed for the prepotency of the healthy 

 parent. Thus, a plant which begins to show deterioration by one or 

 more of the stamens becoming petaloid, if it be crossed by a healthy 

 "single" flower, will produce offspring with single flowers only. 



17 Mr. Romanes, on reading my MS., has appended the following mteresting 

 observation : — " I agree that Weismann is wrong in saying that cessation of 

 selection must always, and in the first instance, produce atrophy. What it must 

 always produce is degeneration. But the degeneration may, first of all, have reference 

 to the breaking down oi organisation (mutual co-ordination of parts, &c.) which had 

 been previously formed and maintained by selection. Now, the breaking down of 

 organisation need not entail atrophy, and is even compatible with the hypertrophy 

 of some of the previously co-ordinated parts. The whole apparatus being thrown 

 out of gear, nutriment before supplied to the vanishing structure may be supplied in 

 excess to the others, and all kinds of mal-correlations of growth ensue. In a state 

 of nature, however, the reversal of selection will prune away all such useless 

 growths." 



