ZOOLOGY — TOWER. 287 



species has been both by slow accumulated variations and by muta- 

 tion ; but there is no sharp line of demarkation between the two. 

 In Leptinotarsa no variations are promiscuous or indeterminate. 

 All exist in a few directions only, and the direction of fluctuating 

 variation and of specific differentiation coincide. 



New or modified characters follow in the main Mendel's laws of 

 inheritance ; but some apparently non-Mendelian characters may on 

 further analysis be found to be Mendelian. In inheritance and 

 evolution in this genus of beetles, characters behave as units, and, 

 although a given ' ' unit character ' ' may be composed of several 

 elements clearly distinguishable, the combinations of these elements 

 in a " unit character " seem to predominate. 



In the genus Leptbiotarsa evolution has been determinate and 

 heritable modifications have been produced only in those directions 

 which are compatible with the morphology and physiology of the 

 generic race, and the}^ have been brought about by the direct action 

 of environmental factors upon the germ cells (germ plasm), and by 

 no other means. Specific differentiation takes place by slowly accu- 

 mulated small variations, by rapidly accumulated large variations, 

 and by sudden steps or mutations. The preservation of a newly 

 appearing modification has been in accordance with the Mendelian 

 laws, many pure races having thus been produced ; and the per- 

 sistence of the character in nature is determined by natural selection. 

 Both adapted and unadapted modifications have been studied. How- 

 ever, many, perhaps most, of the specific characters in Leptinotarsa 

 are neutral ; that is, they do not appear to be directly nor even 

 remotely useful or harmful to the species, and hence do not come 

 under the operation of the law of natural selection. Selection can 

 not, in any of its forms, account for the evolution of » the genus 

 Lcptifiotai'sa. 



Extensive experiments have also been carried on in transplanting 

 species of Leptinotarsa from one habitat to another, and the conclu- 

 sion has been reached that a species of conservative nature and 

 narrow habitat is not capable of a successful widespread migration 

 or accidental distribution; nor does it greatly increase in variability 

 under changed conditions. Of decided importance is the clearly 

 demonstrated fact that the variable species increase in variability 

 under changed conditions, and of the highest importance is the 

 increased production of inheritable discontinuous variations under 

 such conditions. 



Various phases of the variability of Leptinotarsa have also been 

 studied, normal fluctuating, place, geographical, and discontinuous, 

 and the completed work has been prepared for publication. 



