17G THE CANADIAN ENTOMOLOGIST. 



TOWER'S EVOLUTION IN LEPTINOTARSA. 



BY FRANK E. LUTZ, COLD SPRING HARBOR, N. Y. 



One of the most important of the recent studies of evolution, and 

 probably the most important of the purely entomological works on this 

 subject, is Wm. D. Tower's "Investigation of Evolution in the Chrysomelid 

 Beetles of the Genus Leptinotarsa."* " In this contribution have been 

 brought together data concerning evolution in the genus Leptinotarsa, 

 Stal, as gathered from various sources [during ii years], and in as far as 

 it applies to the origin of species. In general, the evidence herein pre- 

 sented has been derived from three sources: (i) its natural history, 

 including distribution and cacology, variations, habits and instincts: (2) 

 development ; (3) experijnent." It is the large numl)er and thoroughness 

 of the experiments which makes the work so valuable, and such a refreshing 

 change from the numerous discussions of pin-stuck data that encumber 

 but do not greatly elucidate the problems of evolution. 



Chapter I is an interesting discussion of the geogra[)hical distribution 

 of the genus. By the use of four of the criteria given by Adams (Biol. 

 Bull., 1902), the centre of origin of the genus is found to be Southern 

 Mexico. The other six criteria are rather severely criticised. If space 

 permitted, these might be profitably discussed, as some of the criticisms 

 do not seem to be fully justified. Valuable detailed data concerning the 

 spread of the Colorado potato beetle are collected and given here. 



Chapter II is a study of variation. A number of laws for the genus 

 are deduced. Variation is found to be determinate. " In the elements 

 of the colour " attern there is a tendency for the spots to spread out or 

 contract peripherally, and the stripes and bands to extend or contract at 

 their ends. The spots, stripes and bands are most variable in the posterior 

 or distal i)ortions of the structures on w-iiich they occur, and least variable 

 in the anterior and proximal portions thereof. Increase of pigmentation or 

 modification of colour pattern moves caudalward or distalward, while 

 decrease moves cephalward or medianward." " Large or extreme varia- 

 tions are determinate, and always occur in directions corresponding to the 

 maximum lines of fluctuating variations." "All variations of colour and 

 structural characters are strongly correlated, .... so that causes 

 which produce a variation in one part bring about either directly or 

 indirectly corresponding variations in other parts." 



*Carneg-ie Institution of Washingfton, Publication No. 48, Papers of the 

 Station for Experimental Evolution No. 4. 

 May, 1907 



