ii4 SCIENCE PROGRESS. 



biophor, as soon as one tries to lay hold of it in one shape 

 it appears under another ; but the point to which I wish to 

 draw attention is not this, but the fact that the biophor is 

 credited with all the vital capacities of the cell, and that 

 in explaining the vital phenomena of cells by reference to 

 biophors, the difficulties are only moved further back ; the 

 vital activities of biophors are still unexplained. The ex- 

 istence of ultimate vital units could be affirmed with much 

 greater certainty if we were able to infer from their assumed 

 structure, or their motion relative to one another, that certain 

 consequences must follow, and to demonstrate that such con- 

 sequences do follow as a matter of fact. It is the possibility 

 of doing this which gives so high a value to the atomic 

 theory, but we are far from being able to do it in the case 

 of the hypothetical vital units. Weismann makes the 

 attempt, but he is obliged to confess that "as long as we 

 know practically nothing about the forces which act in and 

 among biophors, it will be impossible to offer an explanation 

 of (a mechanico-physiological) kind ". 1 



The evolutionary theories which have lately been put 

 forward are not, therefore, of the nature of a general 

 statement of fact, but are assumptions made in order to 

 explain the causes of observed phenomena ; they are de- 

 pendent upon reason, not on observation, and they differ 

 from the analogous theories of the last century chiefly 

 in this, that the latter were founded in the first instance 

 on an erroneous observation — that of Malpighi — whereas 

 the former are inferences from a very large number of 

 accurate and verified observations. There is naturally a 

 very wide difference between the resulting theories, and 

 the probability of the latter-day hypothesis is antecedently 

 of a very much higher order than that of the older, although 

 the underlying idea is the same in both cases. The argu- 

 ment for the new evolutionary theory may briefly be stated 

 as follows. In the development of any multicellular organ- 

 ism the ovum is observed to go through a great number 

 of typical changes in typical succession. By the sub- 



1 Op. at., p. 84. 



