178 The Mechanism of Evolution- in Leptinotaksa 



not segregate, but are fixed from the start. This experience is projected back- 

 wards from that point to a hypothetical difference in the gametes, which is not 

 put to test in the effort to discover whether it is capable of dissociation and thus 

 determine its nature. Uniformity of action in succeeding generations has been 

 the test of the constitution of organisms in the past ; but, as is increasingly shown 

 by the work of the Mendelian hybridologists this is not true, and in this work the 

 crossing of species has shown us both of the main sorts of crossing reactions that 

 De Vries and the older authors distinguished in the work of hybridizing 

 organisms. 



In the course of this investigation I have had under cultivation and crossed 

 many species from nature, and as the collective result of this experience, some 

 of which is given here, I must conclude that there are not two main kinds of types 

 of crosses, but that in the crossing of organisms there is evidence of only one 

 type of reaction, namely, that the union of the gametes in fertilization results in 

 the interaction of the two gametic systems in F^, in such fashion as their struc- 

 tures and compositions permit, to produce the resultant adult zygotes, and which 

 in gametogeuesis produce, through the action of the two intermixed gametic 

 systems, separations of these with or without dissociation and interchange of 

 larger or smaller associated groups of gametic factors. 



Examination of the data presented in the description of the different crosses 

 shows, as for example in the simplest series, the crossing of diversa and signati- 

 collis, that from the same parent stocks different types of reaction were obtained ; 

 sometimes the reaction was such that it would be described as the bisexual, at 

 others it was only the imisexual, and at still other times it was intermediate. 

 The parents are quite distinct, do not live in like habitats, and are not completely 

 fertile, but their interfertility is easily altered within limits. It is shown in 

 these experiments to what the mechanism of this difference in the reactions of 

 crossing are due and hoAv they may be produced in experiment. It is further 

 shown that the pure-breeding lines that arise in F^ are capable of being in some 

 instances broken up and their composition determined, showing that they were 

 in actual composition heterozygous, but through the action of the gametic agents 

 the relation of the factors in the gametes was so altered that it persisted, giving 

 the appearance of a pure race, because its successive generations were alike when 

 bred on without test. Proper conditions readily dissociated it, showing its heter- 

 ozygous nature, which was also strongly indicated by the trimodal polygons of 

 the fraternity. In species crosses in which the two or more types of reaction are 

 observed, if when analyzed show that the principle of reaction is the same in all 

 aspects thereof, we may well speculate as to the probable nature of many untested 

 series in which the only test applied was that of their " breeding true." The 

 point has been reached where the only indication that " breeding true " gives is 

 that it tells little of the composition of the line that breeds true, and as a result I 

 have distinguished in this work very carefully between homozygous in gametic 

 constitution, and homozygous in actions. The two may or may not be present 

 in the same line. 



In these experiments many lines arise that are heterozygous in composition, 

 but have been homozygous in action, and it has been shown that some of these 

 can be proven to be such by the proper tests, either complete or in part, while 

 a few lines have with present methods and knowledge resisted the effects to 

 dissociate them in tests. These lines, which have resisted the efforts to dissoci- 



