58 REPORTS ON INVESTIGATIONS AND PROJECTS. 



demonstrated that special observations must be made for the purposes of 

 experiments of this kind. 



During 1910 and 191 1 the introduction of species of Leptinotarsa from 

 different regions of the American continent has been continued. At Tucson, 

 with its typical subtropical desert, all introductions thus far made act in the 

 same manner ; that is, the ability of the introduced race to survive is not a 

 question of food or enemies, but the attainment at some time during the year 

 of the proper complex which will allow the introduced form to breed and 

 permit the progeny to attain their growth. It is found that the different 

 introduced species, coming from a wide range of original habitats, are alike 

 in this respect, and food, enemies, and relation to other activities are minor 

 in importance ; most vital is the ability to breed, and secondly to pass the dry 

 or winter season in successful suspension of active metabolism, in partial or 

 complete hibernation. The breeding activity is in reality the central factor in 

 determining the survival of the introduction, and this depends entirely upon 

 the physical environment, and at this station the water relation is the chief 

 efficient factor in the complex. In all of these organisms there is a very 

 exact and permanent balance for each species ; and, while all are in principle 

 the same, the response to the water-content of the physical environment is a 

 narrowly fixed minimum, characteristic and exact for each species. This is 

 also a fully inherited character, which is alternative in its behavior in crosses. 

 This relation in the introduced species often causes curiously irregular be- 

 havior. They may have to wait for some months, or even for a year or 

 more, before the proper complex is reached, and while a close approach to 

 the proper state will often produce the beginnings of reproductive activity, 

 these are not completed unless the proper state is attained. Many of these 

 false starts are made in some cultures only to result in partly developed eggs 

 and sperm, which then are resorbed, or frequently produce derangements 

 fatal in effect. 



Nearly as important in determining the fate of these introductions are the 

 relations to the water-content needed for the passing of the winter or dry 

 season, which often causes the elimination of a large part of the population, 

 or sometimes all of it. At Tucson the water relation, as might be expected, 

 is the chief factor in the hibernation activities, and is of vital importance to 

 the organism in directing the outcome in the struggle for existence ; but this 

 relation would not of necessity be an efficient modifying factor in the further 

 evolution of the race. The fact that the breeding activities of the race do 

 not take place until a certain condition of the environment is attained would 

 in many introductions destroy all chance of evolution due to changed envi- 

 ronment, so that the mere fact of introduction into a new environment is 

 not always a factor of evolutionary importance. 



Direct evidence that changed conditions of existence do produce inherit- 

 able modifications in animals, both in nature and in experiments in the labora- 

 tory, was most clearly shown in a series of experiments brought to a focus 

 in the past two years. A pure stock of L. diversa was taken as the basis ; a 



