9-1 The Mechanism of Evolution in Leptinotaksa 



It is not of any interest to present or even record the " dominant " and 

 " recessive " " characters " and the arrangement of them in schemes of their 

 " epistatic " or " hypostatic " relations, bnt my object has been the development 

 of an understanding of the structure of the gametic system, the nature of the 

 zygotic reaction complex (soma), and the basis of the operation of the different 

 agents involved and their relation to internal and external conditions, as an aid 

 in the further investigation of development, transmutation, and evolution in 

 these organisms when used as test objects. In the investigation, characters 

 that are "morphological," those that are "physiological," have been investi- 

 gated, although I can not decide where one " class " ends and the other begins ; 

 both in their productive agents and in operation are the same in all respects, 

 save in the resultant manifestation in the living mass. 



LEPTINOTARSA SIGNATICOLLIS X LEPTINOTARSA DIVERSA. 



Crossing of these two species has given the simplest condition that I have 

 found in the crossing of natural species. As shown in Chapter II, the two 

 species are similar in many of their characters, especially in the juvenile stages ; 

 the most conspicuous visible difference is the presence in L. signaticollis of the 

 elytral stripes and their absence in L. diversa. To this must be added the 

 specific difference in general appearance, shape, and size, which is best deter- 

 mined and expressed in biometric determinations of the form index. Nowhere, 

 as far as is known, do the two species live in the same habitat or in the same 

 environmental complex, L. signaticollis living in the Eio Balsas Valley on its 

 eastern side, L. diversa along the edge of the Mexican Plateau on both the east 

 and west sides. My materials for experiment with L. signaticollis Stal all came 

 from the Rancho Basoco location near Cuernavaca, State of Morelos, L. diversa 

 n. sp. from the Cerro Borrega colony at Orizaba, State of Vera Cruz, the two 

 separated by the high, dry, plateau of southern Mexico, where life is impossible 

 to both species. 



Stocks obtained from these locations, taken to the laboratory, were reared 

 under the same conditions for 4 to 6 generations, and then utilized in the 

 experiments first described. No effect resulted from the change from nature to 

 the laboratory conditions except altered rates of growth, both living in perfect 

 health, giving vigorous offspring in large numbers, in long, continued series, 

 with the same growth-rates after 4 to 6 generations. 



THE NORMAL REACTION IN CROSSING. 



When these two species are crossed under the conditions of the breeding- 

 quarters, between stocks that have become adjusted to like conditions with the 

 same rates of development, a most regular and stereotyped series of results are 

 obtained, irrespective of the direction of the cross, age of the parents, and 

 strength or vigor of the mated pair. Crosses of old and young are completely 

 without effect upon the progeny, as far as any discoverable effect upon either 

 characters or reactions is concerned, nor is there any indication that the gametes 

 show changes in constitution or reaction with age, or changes corresponding to 

 or correlated with ontogenetic changes in the parent of any sort. I made 

 especial effort to check this aspect of the crossing, that I might eliminate the 

 " interpreting " of irregularities that might be found as due to ontogenetic or 



