Botany and Zoology. 



jnes evidently due 



That some variation is atavism is clear enough. This is the 

 natural explanation of the appearance of characters wanting in 

 the immediate parents but known in their ancestors or presumed 

 ancestors. But the assumption of hypothetical ancestors to ac- 

 count for variation generally is quite another thing. Besides its 



bility as an hypothesis is set in a strong light by Naudin's own 

 forcible conception of the nature of heredity. What is heredity ? 

 he asks. In other words, what keeps species so true, ofFspiing like 

 parent, through the long line of generations? He illustrates 

 hereditary force by comparing its action with that of physical 

 force, in which the movement from one state of equilibrium to 

 another is always that in which there is least resistance. From 

 which it follows that when it has once begun to proceed in a cer- 

 tain course, its tendency to continue in that direction increases, be- 

 cause it facilitates its way as it overcomes obstacles. In other 

 words this line becomes fixed by habit; vires acquirit eundo ; 

 the stream deepens its bed by flowing ; and the more remote the 

 commencement of a certain course, the more fixed its directi(>n, 

 and the greater its power of overcoming opposition. The speties 

 18 kept true in its course by the sura of the heredities wliirii |-n<s 

 each individual forward in its actual direction. S<> tii.it. as 

 Naudin remarks, if we could calculate the energy witli which 



line onward in the same direction, we should better apprehi-nd the 

 persistence of species, and feel the great improbability that the 

 stream will ever escape from its ancient and well-worn bed, and 

 strike into new courses. 



Xow, in the first place, the more li\ i-ly the concei^ion we fhii« 

 form of the invariability of species, thmuuli :i happy itictai'}!<>ii<-al 



the less the possibility of its turninLT l^ack \\\nn\ it-clf. and rrMirn- 

 ing old characteristics. The e.Mic- <.f ata\i>-ni irlic ic-umpti-.n of 

 dropped characters) are not lik.lv To . '\tc. id Lack vnvfar; and 

 It seems gratuitous to have recourM- r.. them in explanation oi ,u-%v 

 fornis. Moreover, althonirh the ^tnani ha- maJe ir- he.j aii^i lies 

 "» it, not escaping fn.ni iiZ <,nmi xmHcv. it h tlexil.le . i.oULdi to ul)- 



Like Agassiz. Naudin i-onccivo < 



