424 EVOLUTION AND GENETICS 



they might definitely influence the heredity of the species. The 

 hypothesis is so unsatisfactory that it is of no more than historical 

 interest. The assumption of purely hypothetical structures for 

 which absolutely no biological support is available is never a 

 convincing basis for scientific thought. 



The Mneme Theory. More recently Semon proposed the mneme 

 theory, which is based on the supposition that organic activity 

 and conditions surrounding the organism leave definite traces in 

 the form of engrammes. The engrammes are supposed to accumu- 

 late if the condition producing them is continued, and to affect 

 heredity. It is similar to pangenesis in proposing a purely hypo- 

 thetical vehicle for the recording of the character of the organism, 

 and so is of no more value. 



Centro-epigenesis. Rignano's theory of centro-epigenesis is 

 expressed in an ample and excellent treatise on the inheritance of 

 acquired characters. In this work, for the first time, we seem to 

 be very near to a logical consideration of the factors involved. 

 Rignano emphasizes the importance of functional activity in the 

 organism in connection with environmental stimuli as the cause 

 of evolutionary change. His analysis of characters which have 

 erroneously been considered as disproof of Lamarckian views and 

 those which are really entitled to consideration is logically admir- 

 able and his work seems worthy of more general consideration 

 than it has received. 



Kinetogenesis. A third theory, proposed by Cope, is of limited 

 application. It is fundamentally similar to the last in that it is 

 based upon power of functional response to environmental con- 

 ditions. This theory, in brief, assumes that the effects of repeated 

 motion or impact, as in the striking of the feet of cursorial animals 

 upon the ground, finds expression in greater development of the 

 parts involved. It would account for the development of elongated 

 feet in the unguligrade animals in this way. However, it has 

 been shown that stimuli of pressure may result in the limitation 

 of bone development, as well as increased deposition of bone, so 

 that the theory is applicable to both of the opposed processes. 

 It may be looked upon as one of the valuable subsidiary theories 

 of evolution, although it is distinctly Lamarckian and to that 

 extent is without actual i:)roof. 



The Value of Lamarck's Theory. The foregoing pages are a 

 brief statement of the variegated career of the Lamarckian theory 



