WEISMANN's theory of lIKIiEDITY. 443 



Seein,!? that according: to tlic theory, it is only congenital variations 

 wliich can be inherited, all variations subsequently acquired by the in- 

 tercourse of individuals with their environment, however beneficial such 

 variations may be to these individuals, are ruled out as regards the 

 species. Not falling within the province of heredity, they are blocked 

 off in the first generation, and therefore present no significance at all 

 in the process of organic evolution. No matter how many generations 

 of eagles, for instance, may use their wings for purposes of flight; and 

 no matter how great an increase of muscularity, of endurance, and of 

 skill, may thus be securtMl to each generation of eagles as the result of 

 individual exercise; all these advantages are entirely lost to progeny, 

 and young eagles have ever to begin their lives with no more benefit 

 bequeathed by the activity of their ancestors than if those ancestors had 

 all been barn-door fowls. Therefore the only material which is of any 

 count as regards the species, or with reference to the process of evolu- 

 tion, are fortuitous variations of the congenital kind. Among all the 

 numberless congenital variations, within narrow limits, which are 

 perpetually occurring in each generation of eagles, some will have 

 reference to the wings; and although these will be fortuitous, or occur- 

 ring indiscriminately in all directions, a few of them will now and then 

 be in the direction of increased muscularity, others in the direction of 

 increased endurance, others in the direction of increased skill, and so on. 

 Now each of these fortuitous variations, which happens also to be a 

 beneficial variation, will be favored by natural selection ; and because 

 it likewise happens to be a congenital variation, will be perpetuated by 

 heredity. In the course of time, other congenital variations will happen 

 to arise in the same directions ; these will be added by natural selection 

 to the advantage already gained, and so on, till after hundreds and 

 thousands of generations the wings of eagles become evolved into the 

 marvelous structures which they now present. 



Such being the theory of natural selection when stripped of all rem- 

 nants of so-called Lamarckian principles, we have next to consider what 

 the theory means in its relation to germ-[)lasm. For as before ex- 

 plained, congenital variations are supi)()sc(l by W'eismanu to be due to 

 new combinations taking place in the germ-plasm as a result of the 

 union of two complex hereditary histories in every a(;t of fertilization. 

 Well, if congenital variations are tiius nothing more than variations of 

 germ-plasm " writ large " in the organism which is developed out of the 

 plasm, it follows that natural selection is really at work upon these 

 variations of the germ-plasm. For although it is i)roximately at work 

 on the congenital variations of organisms after birth, it is ultimately, 

 and through them, at work upon the variations of germ-plasm out of 

 which the organisms arise. In other words, natural selection in i)ick- 

 ing out of each generation those individual organisms which are by 

 their congenital character best suited to their surrounding conditions 

 of life, is thereby i^icking out those peculiar combinations or variations 



