457] COLORS OF TIGER BEETLES— SHELFORD 63 



may be due to advantageous nutrition conditions arising from the mor- 

 pliology of the wing, or to special characteristics of the cells themselves. 



BEARING OF THE COLOR PATTERN MECHANISM ON ORTHOGENESIS 



Orthogenesis is commonly understood as evolution in certain direc- 

 tion as opposed to evolution due to the survival of certain kinds of 

 variations out of a large fortuitous, series. The chief points in the 

 original contention of Eimer, namely, that progress in species formation 

 has been along definite lines, has been so generally admitted that the 

 remaining matters are concerned with such questions as: Hoav definite 

 have the directions of modification been? What are the causes of cer- 

 tain-directions of modification being developed to the exclusion of 

 others? Are the causes external or internal? Whitman has empha- 

 sized the internal causes, which is the tendency of all who come at the 

 problem from the point of view of embrj'ology, cytolog.y, and modern 

 genetics. The mystical nature of the question of the origin of a com- 

 'i \ plex organism from a single cell, transmitted through the egg and the 

 ' / sperm of tlie entire series of details which are inherited, have fascinated 

 men and led to the general acceptance of theories which' involve the 

 insulation of the bearers of herditary characters from the environment. 

 The evidence at hand does not justify any detailed discussion of this 

 problem but I will turn to the few things which appear to apply to 

 the tiger beetle group. 



The effects of high temperature on tranqucbarica produce varia- 

 tions in the direction of shortening the longitudinal portion of the 

 middle band and throwing this marking into an oblique position. This 

 is also one of the general tendencies in a large group of tiger beetles. 

 In tranqufharica it occurs as a response to stimulation, and in its races 

 of luiknown stability in regions in which high soil temperatures may 

 be expected. It occurs in nearly half the species of the group of tiger 

 beetles as a regular, probably hereditary character. Tlie condition of 

 the middle band seems to be due to a mechanism of response or modi- 

 fication, which is the same in these responses to stimuli and in the 

 regular heredity trends. The problems of herdity then appear to be 

 the same as the problems of development and modification of this 

 elytral character of Cicindela. Perhaps the weakest point in the en- 

 tire method of study and reasoning of tliose interested in problems of 

 heredity is tlie apparent practical assumption that laws of heredity 

 are not the same as laws governing characters, in particular organs, 

 and as laws of response. Tlie evidence presented tends to show that 

 these laws are one and the same and are dependent upon a mechanism 

 present in the elyti*a of many species of Cicindela. If tliis is what is 



