time: the refreshing river 



Nevertheless, experimentation, the active interference with the 

 course of nature and the subsequent observation of the resulting 

 system in comparison with systems in which no such interference 

 has taken place, was a characteristically nineteenth-century product 

 as far as biology and embryology are concerned. Only at the present 

 day, indeed, are we beginning to appreciate the statistical and other 

 difficulties attending upon the full application of the experimental 

 method to living organisms, and the manifold obstacles which prevent 

 obedience to the rule that only one variable be modified at one time. 

 But this is no matter of reproach against the older embryologists. 

 Knowledge of form, must necessarily precede knowledge of change of 

 form and the factors producing it, and so we see during the last 

 seventy years the production of "Normaltafeln" or tables of morpho- 

 logical pictures showing normal development; these are the essential 

 basis for experimental studies. 



On the other hand, there can be no doubt that a plethora of observa- 

 tion and experiment is also bad for scientific progress. Modern biology 

 is the crowning instance of this fact. What has been well called a 

 "medley of ad hoc hypotheses" is all that we have to show as the 

 theoretical background of a vast and constantly increasing mass of 

 observations and experiments. Embryology in particular has been 

 theoretically backward since the decay of the evolution theory as a 

 mode of explanation. Embryologists of the school of F. M. Balfour^ 

 thought that their task was accomplished when they had traced a 

 maximum number of evolutionary analogies in the development of 

 an animal. Wilhelm His,^ perhaps the first causal embryologist, 

 struggled successfully to end this state of affairs. "My own attempts," 

 he wrote in 1888 in a famous passage,^ "to introduce some elementary 

 physiological or mechanical explanations into embryology have not 

 been generally agreed to by morphologists. To one it seemed ridiculous 

 to speak of the elasticity of the germinal layers: another thought 

 that by such considerations we put the cart before the horse; and one 

 recent author states that we have something better to do in embryology 

 than to discuss tensions of germinal layers, etc., since all embryological 

 explanation must necessarily be of a phylogenetic nature." But this 

 strictly evolutionary dominance in embryology did not last on into 

 the twentieth century. The unfortunate thing is that nothing has so 

 far been devised to put in its place. Experimental embryology, 



^ Comparative Embryology, 1880. ^ Unsere Korperform, 



^ Proc. Roy. Soc. Ed., 1888, 15, 294. 



