32 The Science of Life. 



the selective force exerted by outward circumstances, I 

 deem an innate tendency to deviate from parental type, 

 operating through periods of adequate duration, to be 

 the most probable nature, or way of operation, of the 

 secondary law, whereby species have been derived one 

 from another ". 



Apart from the results of anatomical analysis, though 

 really inseparable from these, the greatest service which 

 Owen rendered to the science of morphology was his 

 clear definition of homology and analogy (1843), the for- 

 mer being illustrated by "the same organ in different 

 animals under every variety of form and function" (e.g. 

 fore-limbs of Draco volans and wings of Bird); the latter 

 being illustrated by "a part or organ in one animal 

 which has the same function as another part or organ in 

 a different animal" (e.g. parachute of Draco and wings 

 of Bird). In other words, organs of similar function are 

 analogous, organs of similar structure and development 

 are homologous. 



The conception of homology was worked out in greater 

 detail by Owen, but we cannot discuss it, nor its further 

 elaborations by Agassiz and Bronn, Haeckel and Mivart. 

 The most important modification is due to Lankester, 

 who, in 1870, distinguished homogeny, or correspondence 

 due to common descent, from homoplasty, "that close 

 agreement in form which may be attained in the course 

 of evolutional changes by organs or parts in two animals 

 which have been subjected to similar moulding condi- 

 tions of the environment, but have no genetic commu- 

 nity of origin to account for their close similarity in 

 form and structure ". 



Although we rank Huxley (1825-1895) among the 

 morphologists , it was not in this capacity that he left 



Huxie ki deepest mark on British biology. For his 

 influence mainly depended on the fact that he 

 combined in extraordinarily high development the scien- 

 tific and the practical mood. In illustration of Huxley's 

 scientific mood we may refer to the high ideal of accu- 

 racy which characterized his work and writings, and 

 quite as markedly his popular lectures, to the caution 

 which made him so reserved as to any causal theory of 



