250 DR. J. W. HESLOP HARRISON ON 



As may be readily perceived, in this original and restricted 

 sense no explanation was or is advanced as to the cause of 

 the phenomenon, although, of course, speculations as to what 

 were the agents responsible for its existence were absolutely 

 inevitable. From such theorising arose the transferred 

 meaning of the word, in which it is employed, not as just 

 explained to include only observed facts, but to cover the theory 

 or theories evolved to account for those facts. The latter 

 usage of the word is followed by Lull in his book on "Organic 

 Evolution" in which he asserts that "Orthogenesis is a theory 

 that variations, and hence evolutionary changes, occur along 

 definite lines impelled by laws of which we know not the 

 origin." Evidently, too, it is in this sense that Morgan looks 

 on the word ; otherwise he would not have described ortho- 

 genesis as being little more than a " mystic sentiment." 



Such, then, are the differences in interpretation existing in 

 the works of present-day writers, but they are in nowise 

 attributable to Eimer. He very carefully distinguished 

 between his actual illustrations drawn from a wide field in the 

 Lepidoptera to explain the principle, and the original causes 

 thereof. Briefly, without appealing to the inner driving force 

 or principle of Morgan, or the " urge ", vis-a-tergo, " kick " or 

 " vital force " quoted by the same author as synonyms of 

 orthogenisis, Eimer announced that " Die Ursachen der 

 bestimmt gerichteten Entwicklung liegen nach meiner 

 Auflfassung in der Wirkung ausserer Einfliisse — Klima, 

 Nahrung — auf die gegebene Konstitution des Organismus."* 

 By this dictum Eimer, without equivocation, commits himself 

 to a belief in the inheritance of acquired characters. Although 

 I made no definite pronouncement it will easily be deduced 

 from my work on the Geometrid genus Oporabia that I was 

 driven to similar views to account for various facts concerning 

 lepidopterous larvcX. 



* " The causes of orthogenesis, according to iny notion, lie in the iiclion of 

 external influences, such as climate and food, on the given constitutiuu of 

 I he organism." 



