102 EVOLUTION IN COLOR-PATTERN OF THE LADY-BEETLES. 



the species, and since var. caseyi has arisen within the range of H. 

 convergens, this seems to be the most probable explanation, especially 

 since the color-pattern difference is correlated with a difference in size, 

 as shown in table 10. 



Where isolation can be resorted to, an explanation is simple. Determin- 

 ate variation in the isolated groups has worked in different directions, so 

 that the reproductive organs or processes may have often become so diverse 

 as to make them intersterile if a breakdown of the isolation brings them 

 to the test. In this I would suggest we find a reason why great organic 

 differences in artificial varieties do not bring about intersterility, while 

 much slighter differences in nature do. In the latter case determinate 

 variation has had its opportunity; in the former it has not. In these 

 beetles, however, isolation can only rarely give this assistance. 



SUMMARY OF CONCLUSIONS. 



(1) Variation. Both continuous and discontinuous variations are found 

 in the color-pattern of these beetles. Variations are also found disposed 

 around certain centers of variation in greater numbers. Yet these centers 

 lack the discreteness necessary to constitute them unit-characters. 



(2) Modification. The color-pattern is capable of modification by the 

 environment. Some modifications exist as hereditary characters also, 

 while others do not. Non-hereditary modifications are more controlled by 

 the structure than are the hereditary variations. The germ-plasm and the 

 soma are in some cases capable of parallel modification, thus producing an 

 apparent inheritance of somatogenic characters. 



(3) Distribution. The species overlap to a great extent. The varieties 

 occur with the typical species in a part of their range. Jordan's law is in 

 general not followed; hence the evolution is probably for the most part 

 not by the even flow of all the individuals in a region. 



(4) Heredity. Segregate (alternative) heredity is general, but it varies 

 by degrees from blending to a typical segregate heredity. Mendel ian 

 interpretations meet with difficulties in most cases. It is probable that 

 we have preponderance (prepotency of characters) in some cases. 



(5) Phylogeny. There does not seem to be adequate ground for postu- 

 lating a definite single pattern as the primitive one for the family. Eimer's 

 laws of pattern development are not applicable. Several congeries of vari- 

 eties or species of diverse patterns may be attributed to descent from a 

 spotted pattern. 



(6) Evolution. Natural selection, if at all active, is principally conserva- 

 tive of the spotted pattern. In spite of this, determinate variation, largely 

 actuated by the effect of the environment on the germ-plasm, and probably 

 preponderance as well, have accomplished marked evolution of the pattern 

 from this condition. Evolution proceeds by waves as well as by even flow 

 and by mutation in different characteristics at different times. 



