CELL-DIVISION AND DEVELOPMENT. 145 



division more than is proper would produce a monstrosity, yet 

 through thousands of generations and thousands of individ- 

 uals the number of divisions is just sixteen ! Can we imagine 

 the physical forces which determine this most remarkable regu- 

 larity ? It is, again, the final result which determines the 

 division, but to say that, does not carry us far enough. There 

 are intrinsic forces at work far too complicated for our under- 

 standing at present, and to speak of the phenomenon as pro- 

 duced by physical causes conveys an idea to our minds but lit- 

 tle more definite than that which we get by speaking of a vital 

 force. 



We have heard a great deal lately concerning cell-dynamics, 

 and the reduction of all vital phenomena to the action of molec- 

 ular forces. I wish in conclusion to sound a note of warning 

 to younger students of Biology, lest they be carried away by the 

 apparent simplicity of such theories. I have endeavored to 

 show that none of the mechanical theories so far proposed will 

 account for the arrangement of the cleavage spindles in certain 

 ova, and that our knowledge of the fundamental properties of 

 protoplasm is too scant to enable us to formulate any scheme 

 of molecular interaction sufficient to account for the phenomena 

 of development. It is not to be inferred, of course, that because 

 we cannot do this at present it never will be done. On the 

 contrary, all investigations looking toward a complete knowledge 

 of the molecular constitution of protoplasm, and of the forces 

 at work in it, for it is not only extrinsic forces which must 

 be considered, all such investigations are most worthy of 

 encouragement. 



What is to be protested against, however, is the tendency to 

 overlook the intrinsic forces, which are of far greater importance 

 than the extrinsic. They may, it is true, be largely of the 

 same nature as the forces which act from without, but they 

 require study, for yet we know practically nothing concerning 

 them. To ascribe vital phenomena at present to the action of 

 molecular forces is to recall the speculations of the classical 

 philosophers who found in the primary elements, earth, fire, air, 

 and water, and their attributes, all the materials for a cos- 

 mogony. We must adopt an analytic method in dealing with 



