Febbuary 11, 1910] 



SCIENCE 



203 



have been born to perish, what has given 

 us the progressive chaiios of beings? 

 Chance, says one extreme view; purpose- 

 ful response, says the other. 



I need not repeat before this body of 

 naturalists that to-day we have dropped 

 entirely the antiquated use of the word 

 chance as something not subject to the laws 

 of mechanics. That conception of chance 

 arose, no doubt, because chance events are 

 those that can not be predicted individ- 

 ually and what he can not predict seems 

 to the confused thinker to disobey the 

 causal law. Out of his ignorance he 

 imagines blind happenings. 



We mean by chance, in ordinary speech, 

 two main things. ' ' I chanced to be there, ' ' 

 we say, meaning that our being there was 

 not connected with what occurred, not that 

 mysterious forces, instead of two legs, car- 

 ried us there. The other meaning is that 

 of a large number of possible combinations 

 a particular one happened. 



Darwin used chance variations as syn- 

 onymous with fluctuating variations. He 

 clearly understood that a chance variation 

 is one due to some unknown cause or com- 

 bination of causes. 



But it is the other sense of the word 

 chance that is of capital import for the 

 matter we have in hand. In this sense 

 chance means that a variation having ap- 

 peared, chanced to find a suitable environ- 

 ment. In this latter sense only is it 

 desirable to use the word chance in connec- 

 tion with organic evolution. The con- 

 fusion of this meaning with the other one 

 which applies to the origin of a variation 

 has led to a regrettable obscurity in the 

 minds of some evolutionists. 



Darwin's famous book is entitled "The 

 Origin of Species" but his theory of nat- 

 ural selection explains the adaptations of 

 living things. Darwin was in a large 

 measure concerned with demonstrating 



that species, in the Linnsean sense of 

 species, arose by evolution, not by special 

 creation. He has himself said : 



Hence if I have erred in giving to natural 

 selection great power, .which I am very far from 

 admitting or in having exaggerated its power, 

 which is in itself probable, I have at least, as I 

 hope, done good service in aiding to overthrow 

 the dogma of separate creations. 



But to-day, accepting evolution, we are 

 concerned as to whether the theory of nat- 

 ural selection explains the origin of species, 

 or whether it explains the adaptations of 

 animals and plants. These two questions 

 have often been merged into one, yet it is 

 notorious that, by systematists, specific dis- 

 tinctions rest in many cases on differences 

 that have no adaptive significance what- 

 ever. 



If, then, the systematist's definition of 

 species is what we mean when we speak of 

 species, and this definition does not con- 

 cern adaptive characters (or only inci- 

 dentally) clearly it is futile to attempt to 

 explain the origin of species by the theory 

 of natural selection. 



Curiously enough, we do, I think, when 

 speaking of adaptation, attach one mean- 

 ing to the word species and another mean- 

 ing when speaking of evolution. In the 

 latter ease we often fall back upon the 

 definitions of the systematist. When we 

 speak of the evolution of adaptations, 

 through natural selection, however, we are 

 thinking of organisms as groups that are 

 structurally and functionally adapted in 

 different ways to the environment in 

 which they live, and differ from all other 

 groups in these relations to the environ- 

 ment. These adaptive characters do not, 

 however, in most cases lend themselves to 

 sharp definition for purposes of identifi- 

 cation and are shunned, therefore, by the 

 systematist. If I am right on this point, 

 the characters of systematic zoology are, 

 at most, only parts of adaptive structures 



