178 Changing Plant and Animal Nature 



than many wild types which are universally recognized as 

 distinct species are derived before our eyes from a common 

 stock. 



The Effectiveness of Selection 



Objection has been raised to the analogy between arti- 

 ficial selection and the natural origin of species on several 

 grounds. Whether natural selection, using Darwin's term, 

 is or is not actually effective, and whether segregation is nec- 

 essary to prevent the swamping of the variants by the pre- 

 ponderance of conforming individuals, may be left for later 

 discussion. More immediate considerations are these facts: 



(i) Domesticated breeds do not remain constant un- 

 less there is repeated selection, and they tend to revert or 

 " throw back " to ancestral types; and 



(2) Domesticated animals and plants cannot maintain 

 themselves unless protected against the exigencies of weather, 

 enemies and disease — they cannot look after themselves in 

 competition with " real " species. 



The first of these objections had considerable weight 

 until experimental methods forced us, since the beginning of 

 the present century, to distinguish between strains that, for 

 various reasons, appear to be distinct, and strains that carry 

 their distinct capacities, so to say, concealed in the germ 

 plasm. About 1903 Villem Johannsen, the Danish botanist, 

 conducted very extensive experiments in the course of which 

 he made some significant discoveries. Selecting a variety of 

 the common bean which is self-pollinating, he carried 

 through successive generations a number of " pure lines." 

 Taking a single bean as the progenitor of a line and saving 

 the seeds from the ensuing plant and growing again from 

 these, he was able in the course of several generations to ob- 

 tain a population of the same ancestry. In such a pure line 

 the smallest seed and the largest seed gave rise in subsequent 

 generations to variable populations, but the offspring of the 

 largest seed varied around the same mode as did the offspring 

 of the smaller seed. There was no tendency in the course of 



