366 PRESENT PKOBLEMS IN EVOLUTION AND HEREDITY. 



the theory of natural selection. Every process is made to suit this 

 theory, which, as we have seen in the first and second lectures, is, in 

 his opinion, the exclusive factor of evolution. But this very high de- 

 gree of mingling" and re-mingling of ancestral predispositions would be 

 fatal to evolution, for after a combination favorable to survival had 

 been estal)lished in one generation it would be broken up into a new 

 combination, x)erhaps unfavorable, to survival, in the next generation. 

 This entire essay upon "Amphimixis," or the theory of mingling of re- 

 duced hereditary substance, will, I believe, mark a turning point to 

 decline in Weismanu's influence as a biologist. His whole reasoning 

 is now in a circle around the natural selection theory. 



The meaning of conjugation. — VYeismann looks upon sexual repro- 

 duction as designed to mingle hereditary tendencies and to create in- 

 dividual differences whereby natural selection may form new species. 

 It is evident that these combinations nuist be mainly fortuitous and 

 l)roductive of indefinite variation; but we have seen that evolution 

 advances largely by the accumulation of definite variations, or those in 

 which each successive generation exhibits the same tendencies to de- 

 part from the typical ancestral form in certain parts of the body, and 

 that these tendencies stand out in relief among the diffused kaleido- 

 scopic or fortuitous anomalies. 



The fact moreover that variability and evolution by the accumula- 

 tion of certain variations in successive generations is also observed 

 in organizisms which reproduce asexuallij, both among plants and 

 animals, shows that we must look in another direction for the under- 

 lying cause or purpose of sexual reproduction. Weismanu rightly com- 

 bats the old idea of " vitalization" of the ovum by the spermatozoon, 

 and it is perfectly evident from the researches of Maupas and Hertwig 

 that the ovum may as accurately be said to vitalize the spermatozoon 

 as the reverse. Fecundation is simply the approximation of two 

 hereditary substances of distinct origin and their incorporation into a 

 single nucleus. The action and re-action of these substances may be 

 considered equal and mutual, so far as we now know. 



The remarkably ingenious ex}>eriments of Hertwig and Boveri, above 

 alluded to, strengthen this idea. Some years ago Weismann wrote: 

 "If it were possible to introduce the female pronucleus of an egg into 

 another egg of the same species, immediately after the transformation 

 of the nucleus of the latter into the female pronucleus, it is very prob- 

 able that the two nuclei would conjugate just as if a fertilizing sperm 

 nucleus had penetrated. If this were so, the direct proof that egg- 

 nucleus and sperm nucleus are identical Avould be furnished." Boveri 

 succeeded in accomi)lishing a similar feat by depriving an ovum of its 

 nucleus and subsequently causing it to develop by admitting a sperma- 

 tozoan which fertilized the denucleated ovum and produced a complete 

 iu dividual. 



In opposing the vitalizing properties of the sperm, Weismann how- 



