"1 



THE FOUR INSEPARABLE FACTORS OF EVOLUTION. 305 



1. That the genesis of many new characters occurs through some internal 

 action in heredity without the initiation or coincident appearance of similar 

 characters in ontogeny. 



2. That such genesis of new characters in heredity occurs not spontaneously 

 nor irrespective of other conditions, nor from the purely internal mechanical 

 forces of heredity, but through some entirely unknown and at the present time 

 inconceivable relation between the forces of heredity and those of ontogeny and 

 environment. 



3. That such characters in heredity arise determinately, definitely, but by 

 extremely slow stages, or continuously. 



4. That animals of different but remotely related lines of descent Bhow degrees 

 of similarity in the genesis of such characters which correspond in closeneoi 

 with the degrees of taxonomic relationship. This is the law of "potential 

 homology 



5. The degrees of taxonomic relationship also affect to a certain exten t the 

 time of the genesis of new characters as well as the rate of devehpmcnt of new 

 characters. 



6. That since such genesis of new characters springs from the internal forces 

 of heredity, the continuous or discontinuous appearance of a new character may 

 be simply the expression of the same law or operating at a different rate. 



The principle of potential homology is based upon the observation that animals 

 of similar kinship do not continuously evolve in certain directions but merely 

 transmit a similar potentiality in the genesis of new characters. This hereditary 

 potential both renders possible the occurrence of certain characters and con- 

 ditions or limits or forms these characters when they do occur. This is not an 

 internal perfecting tendency in heredity through which animals reach a certain 

 condition through the operation of an internal mechanical law of development, 

 but a potentiality in heredity which under the law of the four inseparable factors 



38, July, 1890 (meeting, Toronto, Aug., 1889), pp. 273-276. Rept. Brit. Assoc. Adv. Sci., Newcastle- 

 upon-Tyne (meeting of Sept., 1889), London, 1890j 

 sion of Acquired Characters, Nature, vol. XLI, Jan., 1890, pp. 227-228. Certain Principles of Pro- 

 nely Adaptive Variation Observed in Fossil Series, Rept. Brit. Assoc. Adv. Sci., 1894, p. 693 

 (title). Nature, vol. 50, No. 1296, Aug. 30, 1894, p. 435. The Hereditary Mechanism and the 



Transmis 



gressi 



1895. 



Led. Marine Biol. Lab 

 May, 1895, pp. 418-439 



Problem! 



To-day: Palseontological Problems (Discussion before the annual meeting of the American Society of 



Naturali 



145-147. Evolution as it Appears 



to the Paleontologist. Address before the Seventh International Zoological Congress, Section of 

 Palaeozoology, Boston, Aug. 1907. 



744- 



Proc. Seventh International Zool. Conor., Boston Meeting, Aug. 19-24, 1907, Cambridge, Mass 



1912, pp. 733-739. Darwin 

 8vo. 



One of the Addresses in Fifty Years of Darwinism 



209-250 



Science 



895-896. 



• T.o*oTif «r Pr.font.m1 TTnmnloffv. Amer. Naiuralid 



AAA VI, Apr., 1902, pp. 259-271 

 angular Type. 8vo. Macmillan ( 



Mammalian Molar 



See Chap. X. 



