THE FACTORS OF ORGANIC EVOLUTION. 203 



dial units of protoplasm, then, the step with which evolution com- 

 menced must have been the passage from a state of complete likeness 

 throughout the mass to a state in which there existed some unlikeness. 

 Further, the cause of this step in one of these portions of organic mat- 

 ter, as in any portion of inorganic matter, must have been the differ- 

 ent exposure of its parts to incident forces. What incident forces ? 

 Those of its medium or environment. Which were the parts thus 

 differently exposed ? Necessarily the outside and the inside. Inevi- 

 tably, then, alike in the organic aggregate and the inorganic aggregate 

 (supposing it to have coherence enough to maintain constant relative 

 positions among its parts), the first fall from homogeneity to hetero- 

 geneity must always have been the differentiation of the external sur- 

 face from the internal contents. No matter whether the modification 

 was physical or chemical, one of composition or of decomposition, it 

 comes within the same generalization. The direct action of the me- 

 dium was the primordial factor of organic evolution. 



In his article on Evolution in the Encyclopedia Britannica, Pro- 

 fessor Huxley writes as follows : 



"How far 'natural selection' suffices for the production of species remains 

 to be seen. Few can doubt that, if not the whole cause, it is a very important 

 factor in that operation. . . . 



On the evidence of palaeontology, the evolution of many existing forms of animal 

 life from their predecessors is no longer an hypothesis, but an historical fact; it 

 is only the nature of the physiological factors to which that evolution is due 

 which is still open to discussion." 



With these passages I may fitly join a remark made in the admirable 

 address Prof. Huxley delivered before unveiling the statue of Mr. 

 Darwin in the Museum at South Kensington. Deprecating the sup- 

 position that an authoritative sanction was given by the ceremony to 

 the current ideas concerning organic evolution, he said that "science 

 commits suicide when it adopts a creed." 



Along with larger motives, one motive which has joined in prompt- 

 ing the foregoing articles, has been the desire to point out that already 

 among biologists, the beliefs concerning the origin of species have 

 assumed too much the character of a creed ; and that while becoming 

 settled they have been narrowed. So far from further broadening 

 that broader view which Mr. Darwin reached as he grew older, his 

 followers appear to have retrograded towards a more restricted view 

 than he ever expressed. Thus there seems occasion for recognizing 

 the warning uttered by Prof. Huxley, as not uncalled for. 



Whatever may be thought of the foregoing arguments and conclu- 

 sions, they will perhaps serve to show that it is as yet far too soon to 

 close the inquiry concerning the causes of organic evolution. 



