ACQUIRED CHANGES AND HEREDITY 137 



assumed by every one ; and in part to real difficulties 

 which, however, require the most careful re-examination 

 before they can be accepted as proofs of the transmission 

 of acquired characters and as the death-blow to Weis- 

 mann's hypothesis. 



If the direct evidence for the transmission of acquired 

 characters fails to stand the ordeal of a thorough investi- 

 gation, the indirect evidence still remains. If it could 

 be shown that certain phases of evolution would have 

 been impossible without such transmission, we should be 

 compelled to maintain that the latter had taken place. 



The chief lines of indirect evidence are : — The fact of 

 individual variation, the effects of use and disuse of parts, 

 the facts presented by the phenomena of instinct. 



Individual variation was believed to be due to the 

 hereditary effect of the direct action of environment. It 

 is known that in some cases (e. g. certain plants) variation 

 has been caused by the direct action of environment on 

 the germ-cells while still contained in the body of the 

 parent. Such a change is, of course, blastogenic, and 

 would be transmitted. There is less evidence for the 

 operation of such causes in the case of animals. The 

 consideration of twins and monstrosities pointed to the 

 conclusion that individual variation is predetermined in 

 the fertilized ovum. If it be asked why the germ-cells 

 of an individual should differ among themselves, Weis- 

 mann has pointed out that there is reason to believe that 

 the changes which ova and spermatozoa undergo, as 

 a preparation for their fusion in fertilization, must lead 

 to individual differences. He, therefore, considers that 

 variation is produced by sexual reproduction, and is, in 

 fact, its raisoii d'etre. The meaning of this form of 

 reproduction is to supply variations upon which Natural 

 Selection can operate. 



The apparently hereditary effects of increased use are 

 more probably due to the operation of Natural Selection 

 upon a part which is, ex hypothesis of especial importance, 

 combined with the admitted strengthening and growth 

 which follow increased use during the life of the indi- 

 vidual. The apparently hereditary effects of disuse are 



