CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



839 



tion of any given biocharacter is environmental, 

 somatic, or germinal. If, for example, in all the 

 instances observed the inititation of a certain allo- 

 metron is primarily somatic and secondarily germinal, 

 this fact would be presumptive evidence that in this 

 particular biocharacter somatic action, reaction, and 

 interaction precede germinal predisposition. Even if 

 it were proved that a certain allometron is invariably 

 somatic in its initiation and secondarily germinal, it 

 would not be logical to assume that all allometry is 

 first somatic and then germinal; in fact, we shall point 

 out that certain allometrons are germinal from the 

 first; they are not, so far as we loiow, initiated in 

 ontogeny. Moreover, invariable sequence is not 

 invariable cause and effect. 



BEARING OF SALTATION VERSUS CONTINUITY ON 



THE LAMARCKIAN, DARWINIAN, AND 



TETRAKINETIC THEORIES 



According to the Darwinian theory variations should 

 be observed as chiefly discontinuous, saltatory, fortu- 

 itous, adaptive or nonadaptive, furnishing material 

 for natural selection. 



According to the Lamarckian and the tetrakinetic 

 theories variations shoidd be mainly continuous and 

 adaptive, rarely saltatory and fortuitous. What are 

 the observed facts of variation? 



DARWIN's hypothesis of FORTUITOUS SALTATION 

 AND FLUCTUATION 



After years of observation of domesticated plants 

 and animals under artificial selection, Darwin reached 

 the conclusion that there were three kinds of new 

 variation characters for natural selection to work 

 upon — first, hereditary minor saltations ("minute 

 heritable variations"); second, hereditary fluctua- 

 tions of proportion; third, somatogenic modifications 

 of proportion, which he finally believed (Lamarckian 

 theory) to be inherited. 



Darwin (1859.1) held that evolution is due chiefly 

 to the natural selection of "heritable individual difl'er- 

 ences" — that is, variations — his real meaning as to 

 these individual differences being found in the hun- 

 dreds of examples he cited in the "Origin of species" 

 and in "Variations of animals and plants under 

 domestication." Poulton (1909.1, pp. 49-50) re- 

 marks as to Darwin: "His observation and study 

 of nature led him to the conviction that large varia- 

 tions [that is, major saltations], although abundant, 

 were rarely selected, but that evolution proceeded 

 gradually and by small steps." Plate (1909.1) es- 

 tablished clearly that the "individual differences" 

 of Darwin are practically identical with the "muta- 

 tions" of De Vries. Osborn (1912.362, pp. 76-82) 

 held that Darwin's conception was that evolution 

 develops chiefly through the natural selection of minor 

 saltations; but that Darwin's "individual differences" 



are in the nature of both major and minor saltations, 

 structural or functional, and always hereditary, as 

 is shown in the observations that he assembled in 

 commenting on the genesis of the race horse and of the 

 greyhound, breeds which he used by way of illustra- 

 tion of the genesis of new forms in nature. In con- 

 sidering these breeds he pointed to such suddenly 

 appearing new characters as horn rudiments, tail- 

 lessness, curlmess of the hair, characters which are 

 "discontinuous" in Bateson's sense, "mutations" in 

 that of De Vries. Intermingled with these minor 

 saltations Darwin cited others which are obviously 

 reversional. That he believed chiefly in the accumu- 

 lation of favorable minor saltations there can be no 

 question, but, for the admirable reason that no 

 evidence had been adduced in nature of evolution by 

 major saltations, he rejected the hypothesis, which 

 originated with Geoffroy St. Hflaire, of the appear- 

 ance under certain environmental conditions of en- 

 tirely new types of animals and plants or of new pro- 

 foundly modified organs. 



In brief, Darwin held chiefly that by cumulative 

 natural selection of minor saltations a character could 

 slowly be shifted in an adaptive direction. These 

 minor saltations were in his opinion the fortuitous 

 or chance material out of which nature "selects" 

 its adaptations, utilizing the adaptive and rejecting 

 the inadaptive. He guarded the word "chance," 

 however, by stating that it might merely cover our 

 ignorance of the unlcnown causes of variation. 



Darwin also believed in the natural selection of 

 heritable fluctuations of proportion, as illustrated in 

 his classic rebuttal of the Lamarckian explanation of 

 the mode of origin of the long neck of the giraffe, 

 namely: 



So under nature with the nascent giraffe, the individuals 

 which were the highest browsers and were able during dearths 

 to reach even an inch or two above the others will often have 

 been preserved; for they will have roamed over the whole 

 country in search of food. * * * These slight proportional 

 differences will favor survival and will be transmitted to off- 

 spring. ("Origin of species," ed. of Appleton, 1909, p. 27.) 



Naturally Darwin could not draw such sharp dis- 

 tinctions in the definition of variation either in 

 language or in example as we may to-day, profiting 

 as we do by the 50 years of experiment and analysis 

 that have passed since his time. The chief emphasis 

 in the above passage is in the words "slight pro- 

 portional differences," differences that we now classify 

 as fluctuations or fluctuating variabilities. 



In conclusion, critical reexamination of Darwin's 

 writings leads us to dissent entirely from the influen- 

 tial opinion of De Vries that there was always a doubt 

 in Darwin's mind as to whether (1) the selection of 

 minor saltations or (2) the selection of fluctuations 

 played the larger part in the origin of species. The 

 actual examples that Darwin cited and his repeated 

 emphasis prove that minor saltations like the De 



