GENERAL DISCUSSION. 91 



eral in their color-pattern than are the extremes of Hippodamia spuria 

 where there is no isolation. 



Taylor (1894), Matthew (1908), and Cockerell (1908) have proposed the 

 view of an "active evolutionary center" at which a genus or a higher 

 group has reached its highest development and from which the new species 

 arise and are dispersed. In this way the oldest species are found at a dis- 

 tance from the center. We do have an active evolutionary center for Hip- 

 podamia in the Plateau and Pacific States, but it is doubtful if the varieties 

 of species there produced are dispersed and replaced. They are, for the 

 most part, endemic and have no power to extend their range greatly into 

 conditions quite different from those of their origin. Furthermore, the 

 active evolutionary center is here caused by conditions of the environment 

 capable of altering the germ-plasm rather than by any "intense pressure 

 and competition there," as Taylor asserts of the active evolutionary center 

 of the land snails. 



HEREDITY. 



In spite of the many non-conformable cases presently to be mentioned, 

 many biologists have come to expect Mendelian segregation and dominance 

 whenever segregate inheritance is found, so eagerly does the mind wel- 

 come and embrace any conception of uniform action. In the discussion of 

 the several species we have seen that in only one case do the facts seem 

 conformable where, as in several cases, they fail to conform to the simple 

 Mendelian conceptions in spite of the fact of there being some degree of 

 obvious segregation. For the biologist whose predilections are strongly 

 Mendelian this is of little moment, for he will believe that these cases 

 simply demand a few consistent hypotheses of additional factors, enzymes, 

 inhibitors, coupling, latency, etc., to become quite conformable. Since a 

 rigorous test of most of these subsidiary hypotheses to Mendelism demand 

 very large numbers, greater than are often available in animal experi- 

 mentation, the test is not be made and consequently his comfortable 

 faith remains undisturbed. 



But is it good scientific method to keep protected in this way the faith 

 in the Mendelian behavior of insect characteristics ? When we consider 

 the results of Kellogg upon silkworms (1908), Lutz upon Ampelophila 

 (1908 c) and upon Grioceris (1908 a), McCracken upon Lina (1907) and 

 upon Gastroidea (1906), and Tower upon Leptinotarsa (1906), we find no 

 genus in which there are not such un-Mendelian things as "individual 

 and strain idiosyncracy, " 'variation of allelomorph potency," 'weak 

 factors," 'bilateral opposition of characters," " gradual elimination of 

 alternative characters, " ' progressive dominance, ' ' and ' ' inability to hand 

 on variations with full intensity." 



The least we can do is to give a fair consideration to hypotheses which 

 seem to arise simply from the facts, for, as Chamberlain has well pointed 

 out, the use of multiple hypotheses is a great safeguard to the soundness 

 of scientific progress. 



