of the Costce in the Perisphinctince. 451 



lowers of Darwin and Weismann, for he came ont 

 strongly against the potency of natural selection in the 

 formation of species, considering it only as a process 

 which "may remove what is downright injurious" and 

 "may preserve what is useful" but which is always sub- 

 ordinate to orthogenesis. Furthermore, he advocated 

 the inheritance of acquired characters as a necessary out- 

 come of the operation of the law of orthogenesis and 

 expressed the belief that "the causes of definitely 

 directed evolution [i. e. orthogenesis] are contained . . . 

 in the effects produced by outward circumstances and 

 influences such as climate and nutrition upon the consti- 

 tution of a given organism." 5 



It is not our purpose to enter into a discussion of these 

 causes; we are seeking only to discover how the term 

 orthogenesis should be used, and we see that Eimer, who 

 first gave it prominence, distinguished between the law 

 and the causes of orthogenesis or definitely directed 

 evolution. Later authors, however, have usually con- 

 fused the two. 



Professor Grabau in his ' Studies of Gastropoda' has 

 followed Eimer 's usage, regarding orthogenesis as an 

 observable method of development, and he states that: 

 " Orthogenetic variation may be defined as progressive 

 variation along definite or determinate lines. ' ' 6 



Professor Lull, on the other hand, in his book on 

 c Organic Evolution ' says: "Orthogenesis ... is the 

 theory that variations and hence evolutionary change 

 occur along certain definite lines impelled by laws of 

 which we know not the cause." 7 Finally, Professor 

 Morgan in his ' Critique of the Theory of Evolution' in- 

 cludes the unfolding principle as one of the "four great 

 historical speculations" concerning evolution, stating 

 that "it is little more than a mystic sentiment to the 

 effect that evolution is the result of an inner driving 

 force or principle which goes under many names such as 

 Bildungstrieb, nisus formativus, vital force and ortho- 

 genesis." 8 



From these diverse expressions of opinion it is clear 



6 Loc. cit., p. 22. 



6 Grabau, A. W., 1907. Studies of Gastropoda. On Orthogenetic Varia- 

 tion in Gastropoda, Am. Naturalist, vol. 41, no. 490, p. 607. 



T Lull, B. S., 1917. Organic • Evolution, p. 175. (Macmillan Company.) 



8 Morgan, T. EL, 1916. A Critique of the Theory of Evolution, p. 34. 

 (Princeton University Press.) 



Am. Jour. Sci.— Fourth Series, Vol. XL VIII, No. 288.— December, 1919 

 31 



