200 NATURAL SCIENCE. March. 



each theoretical advance has been of a character intelHgible only to 

 specialists. The essay now before us gives the results of a series of 

 experiments conducted at intervals, since 1875, upon the conditions 

 determining the seasonal and climatic variations of butterflies. The 

 earliest results were translated into English by Professor Meldola, and 

 appeared among the "Studies in the Theory of Descent" in 1882; 

 the present essay has been well translated by Mr. W. E. Nicholson. 

 The existence of these translations is particularly fortunate, as by 

 them many English entomologists may be stimulated to assist in the 

 important work of collecting additional experimental observations on 

 these curious occurrences. Much good work of this kind is well 

 within the opportunities of the ordinary or field collector, and would 

 afford him nearly as much pleasure as the capture of rare duplicates 

 for exchanges. 



The general nature of the circumstances under investigation is as 

 follows : many lepidoptera exhibit, apart from sexual dimorphism, a 

 dimorphism related to climate or to seasons. Some European forms, 

 for instance, have Polar varieties notably different in colour and 

 markings. Some temperate forms have southern, semitropical 

 varieties equally different. Many have two broods ; a spring brood 

 resulting from eggs or pupse that have hibernated, and a different 

 summer or autumn brood. The spring broods tend to resemble 

 northern forms ; the summer or autumn broods resemble southern 

 forms. In Lamarckian days most of these differences were set down 

 naively to the direct effect of heat or of cold. Detailed experiments 

 show that the explanation is not so fortunately simple ; and Professor 

 Weismann devotes his pages to describing his experiments, correlating 

 them with those of others, and discussing their interpretation from the 

 point of view of bionomics and heredity. 



In 1875 1^^ assumed that it was to a certain extent obvious that 

 seasonal dimorphism was the direct result of climatic influences — 

 chiefly of heat and cold. He suggested even then the possibility that 

 the seasonal changes were adaptive, like the seasonal changes of 

 Arctic mammals, with the difference that the changes did not occur in 

 one and the same individual, but alternately in individuals of different 

 generations. However, the upper side of butterflies, which is not 

 usually adaptive, varies as much as the adaptive under side in the two 

 generations : moreover, by artificial subjection to a higher or lower 

 temperature, the stamp of the winter form may be impressed on the 

 summer brood, or vice versa. It seemed plain that the measure of 

 heat acting during the pupal period directly formed the species in one 

 way or in the other. 



Now, however, he is inclined to the view that there is adaptive 

 seasonal dimorphism in addition to direct seasonal dimorphism. There 

 are cases known, for instance, in which one seasonal form differs from 

 another, not merely in colouring, but in the presence of complicated 

 details of marking and colouring, such as ocelli ; and it cannot be 

 thought that heat or cold, drought or humidity, are the direct causes of 

 these. Professor Weismann thinks that in the embryo there are present 

 the germinal constituents for both forms : the seasonal or artificial 

 change acts as the stimulus to liberate and bring into activity one or 

 the other set. In any particular case it may be that both direct and 

 adaptive dimorphism is present. Very many carefully devised 

 experiments, and the fullest observation of the different forms,' alive 

 in their natural surroundings, are required for the discrimination. 

 Professor Weismann discusses his own experiments and those of 

 others so clearly and fully that anyone, with the taste and opportunity, 



