174 The Mechanism of Evolution in Leptinotaksa 



eggs in all of them, most of which developed for short periods, but few of which 

 liatched. Very many die before or at the period of the forming of the limb- 

 buds. The two groups have quite different rates of development, Ac values, 

 and differences in these may be primarily responsible for the death of the 

 embryos. 



It may be possible to produce hybrids between these groups by the altering of 

 the Ac values through the use of external conditions, but thus far the permanent 

 alteration of tl^is character has not been affected by this agency. It is shown in 

 this and the preceding chapters that this character or the agents which it repre- 

 sents are specific for the species, and, while capable of change, has in all thus far 

 tested shown rigid upper and lower limits not transgressed by the action of the 

 pressure of conditions in the medium. 



In one way or another the gametes of the different species in this genus are 

 able to unite and begin development, some to complete it. The problem of the 

 intercrossing of species consists, therefore, of two sets of problems : the initiation 

 of the mating reactions, which are independent of the second, namely, the inter- 

 actions of the gametic systems when placed in the zygote. Evidence is derived 

 from these experiments showing that in the gametes the production of F^ adults 

 is directly associated with rates of development in the two gametic systems, and 

 that likeness of rates is productive of fertility and development, unlikeness of 

 infertility and cessation of development; but there are other at present unde- 

 termined agents present contributing to the infertility and non-development. 

 Many interesting and suggestive openings into this most difficult and important 

 portion of the problems of evolution have been obtained, and I hope to be able to 

 follow them further. 



Suggestive points with regard to the inhibition of the reproductive reactions 

 have also arisen in the course of this crossing, which collectively indicate — in 

 some instances show conclusively — that the reproductive isolation of these 

 species or types is independent of the factors governing the interaction of the 

 gametes and in most instances is an incident in the origination of the line, a by- 

 product of the process through which the form was produced, and I have 

 observed, in the course of this work, instances of this isolation which have arisen 

 in experiment by the reaction of origination, and the isolation has no relation to 

 utility, purpose, or adaptation, nor plays any role iji the origination of the line. 

 It is a purely physical resultant of the reactions that have taken place in the pro- 

 duction of the specific form. In the data of crossing species already given there 

 is positive evidence for this conclusion, while later portions of this report con- 

 tain added data in support of the same proposition. An interesting instance of 

 the rise of a race that is physiologically isolated in different ways is the line that 

 arose from the crossing of L. diversa and L. decemlineata. This race or species 

 crossed with L. decemlineata is slightly fertile, and entirely infertile with L. 

 diversa, L. signaticollis and L. decemlineata, and no method has been found that 

 decreased the infertility in any degree. This was true of the line so long as it 

 was maintained and was the direct and completed product of reaction that gave 

 rise to this race, so that there were produced factorial conditions that effectually 

 inhibited the reproductive activities with the nearest of kin, the parent species; 

 nor has fertility with other species changed as far as has been tested. The condi- 

 tion is not that of either parent, but something new, a fact that is clearly mani- 

 fested by the infertility with both parents. A stock of this character arises as a 



