148 



SCIENCE 



[N. S. Vol. XXVII. No. 682 



SPECIAL ARTICLES 



THE FOUR INSEPAEABLE FACTORS OF EVOLUTION. 



THEORY OF THEIR DISTINCT AND COMBINED 



ACTION EST THE TRANSFORMATION OF THE 



TITANOTHERES, AN EXTINCT FAMILY OF 



HOOFED ANIMALS IN THE ORDER 



PERISSODACTYLA ' 



In a recent address entitled " Evolution as 

 it appears to the Paleontologist"" I promised 

 a fuller exposition of the law of the four in- 

 separable factors. 



During the past six years a very careful 

 analysis of the modes and factors of evolution 

 has been made in connection with my exhaust- 

 ive study of the Perissodactyl family of Titan- 

 otheres, with the following result: aU the 

 processes and modes of evolution should be 

 grouped under the primary processes of (1) 

 heredity, (2) ontogeny, (3) environment, (4) 

 selection. In this grouping heredity includes 

 solely changes in the germ plasm. Ontogeny 

 includes the somatic expression of heredity, 

 somatic modification and adaptation, as well 

 as the somatic environment of the germ plasm. 

 Environment includes all nature external to 

 the organism. Selection represents all com- 

 petition, survival or elimination of individuals 

 representing the combined product of heredity, 

 ontogeny and environment. Variation is not 

 included here because it is a secondary process. 



A survey of the history of evolution theory 

 shows successive waves of opinion or schools 

 holding to the chief or more or less separate 

 influence of these processes as factors; for 

 example, environment (BufPon), ontogeny 

 (Lamarck), selection (Darwin), heredity (the 

 modern school). There is a very large ele- 

 ment both of truth and of error in the tenets 



' Abstract of paper read before the American 

 Society of Zoologists, New Haven, Conn., December 

 26-28, 1907. This address was first delivered to 

 Columbia students, November 3, 1905, and has 

 been held two years for further consideration, 

 before publication. 



' Osborn, Henry Fairfield, " Evolution as it ap- 

 pears to the Paleontologist," Science, N. S., Vol. 

 XXVI., No. 674, No. 29, 1907, pp. 744-749. Ad- 

 dress before Seventh International Congress of 

 Zoology, Section of Paleozoology, Boston, August, 

 1907. 



of each of these schools, because while the 

 influence of each of these factors is undeni- 

 able, the exclusive influence of either of these 

 factors is never found in nature, and can exist 

 only in the mind of the observer. 



The actual state of living nature is that of 

 the inseparahle and continuous action of these 

 several factors as expressed in the following 

 most fundamental biological law : 



The life and evolution of organisms con- 

 tinuously center around the processes which 

 we term heredity, ontogeny, environment, and 

 selection; these have been inseparahle and in- . 

 teracting from the heginning ; a change intro- 

 duced or initiated through any one of these 

 factors causes a change in all. 



Eepresenting these processes respectively by 

 the capital letters H, 0, E, S, life and evolu- 

 tion may be represented by the formula: 



HX O XE X S. 



This formula roughly expresses the intimate 

 nexus which exists between all these processes, 

 a nexus which is quite consistent with the fact 

 that each has also its separate part in life and 

 in evolution. The multiplication sign, Xj ^ 

 to be interpreted in the active and passive 

 sense of influencing and influenced iy. As 

 examples of what is meant by this formula we 

 may cite such principles as the following: (1) 

 that heredity is conditioned by ontogeny and 

 environment; (2) that selection operates on 

 the product of heredity, ont7>geny and environ- 

 ment; (3) that ontogeny initiates many 

 changes which are subsequently taken up by 

 heredity; (4) that of the four processes in- 

 volved in life and evolution heredity is by far 

 the most conservative and stable, among other 

 reasons because it is embodied in a form of 

 matter (germ-plasm) least subject to chan- 

 ging external influences; that ontogeny, on 

 the other hand, is the most unstable. 



In contrast to the graphic representations of 

 the original extreme hypothesis of Weismann, 

 in which heredity is represented as a continu- 

 ous current more or less isolated from ontog- 

 eny and environment there may be presented 

 the following diagram. 



This diagram brings out the real cause of 

 the difiiculties which arise, as illustrated below 



