40 PHYLOGENY 



heredity itself is the only fully reliable proof of heredity. Or, in 

 other words, the average constitution of the offspring is the mark 

 which gives us the information wanted. No visible quality can be 

 a trustworthy substitute for it, and such are only to be regarded as 

 surrogates. 



This average constitution must be expressed by the hereditary 

 percentage of true inheritance of the mark under consideration. 

 If it is determined by the culture of one hundred children of each 

 mother plant, it constitutes the centgener power of that mother, 

 as it is called by Hays. Selection has to rely on this percentage- 

 figure, and the results, already attained, give proof that here a new 

 method is given which is able to yield large and rapid improvements. 

 It is the same principle which since the earliest times is, in the 

 main, ruling the selection of our domestic animals. 



If this principle should prevail and come to exclude the selection 

 on the ground of the visible qualities of the individuals, the com- 

 parison between artificial and natural selection will largely change 

 its aspect. For it is evident that the latent hereditary possibilities 

 of an individual have no influence at all on its chance of surviving 

 in the struggle for life. Only by very remote considerations would it 

 be possible to uphold the significance of this, but in all ordinary 

 cases, this significance for the improvement of the race would be 

 reduced to nothing. 



Thus we see that a close analysis of the factors which provoke 

 the fluctuating variability goes to prove how uncertain the basis is, 

 which it affords for an explanation of organic evolution at large. 

 On many points, artificial and natural intraspecific selection have 

 been compared, but nowhere is this comparison favorable to the 

 current theoretical views. On the other hand, the artificial pro- 

 cesses of variety-testing and the theoretical and presumable selec- 

 tion between elementary species in nature seem to be perfectly 

 comparable. The large practical significance of the first points 

 clearly to an equally large theoretical importance of the latter. 

 Hence we conclude that interspecific selection through the strug- 

 gle for life is the always-acting sieve which keeps evolution on the 

 main lines, and which in this way is the one great Darwinian cause 

 of all organic progress. 



