SCIENCE 



[N. S. Vol. XLI. No. 1044 



ena by deliberate control of the conditions 

 under whicli they take place — the method 

 of experiment. Its steadily increasing im- 

 portance is the most salient feature of the 

 new zoology. 



Experimental work in zoology is as old 

 as zoology itself; nevertheless, the main 

 movement in this direction belongs to the 

 past two decades. I will make no attempt 

 to trace its development ; but let me try to 

 suggest somewhat of its character and con- 

 sequences by a few outlines of what took 

 place in embryology. 



The development of the egg has always 

 east a peculiar spell on the scientific im- 

 agination. As we follow it hour by hour in 

 the living object we witness a spectacular 

 exhibition that seems to bring us very close 

 to the secrets of animal life. It awakens 

 an irrepressible desire to look below the 

 surface of the phenomena, to penetrate the 

 mystery of development. The singular fact 

 nevertheless is that during the phylogenetic 

 period of embryological research this great 

 problem, though always before our eyes, 

 seemed abnost to be forgotten in our pre- 

 occupation with purely historical questions 

 -^such as the origin of vertebrates or of 

 annelids, the homologies of germ-layers, 

 gill-slits or nephridia, and a hundred others 

 of the same type. Now, these questions are 

 and always will remain of great interest; 

 but embryology, as at last we came to see, 

 is but indirectly connected with historical 

 problems of this type. The embryologist 

 seeks first of all to attain to some under- 

 standing of development. It was there- 

 fore a notable event when, in the later 

 eighties, a small group of embryologists 

 headed by Wilhelm Roux turned away from 

 the historical aspects of embryology and 

 addressed themselves to experiments de- 

 signed solely to throw light upon the 

 mechanism of development. The fuU 

 significance of this step first came home to 



us in the early nineties with Driesch's 

 memorable discovery that by a simple me- 

 chanical operation we can at will cause one 

 egg to produce two, or even more than two, 

 perfect embryos. I will not pause to inquire 

 why this result should have seemed so 

 revolutionary. It was as if the scales had 

 fallen from our eyes. With almost a feel- 

 ing of shock we took the measure of our 

 ignorance and saw the whole problem of 

 development reopened. 



The immediate and most important re- 

 sult of this was to stimulate a great num- 

 ber of important objective investigations 

 in embryology. But let me pause for a 

 moment to point out that at nearly the 

 same time a similar reawakening of inter- 

 est in the experimental investigation of 

 problems of the present became evident in 

 many other directions — for example, in 

 studies on growth and regeneration; on 

 cytology and protozoology; on economic 

 biology; on ecology, the behavior of ani- 

 mals and their reactions to stimuli ; on he- 

 redity, variation and selection. The leaven 

 was indeed at work in almost every field of 

 zoology, and everywhere led to like results. 

 It was a day of rapid obliteration of con- 

 ventional boundary lines; of revolt from 

 speculative systems towards the concrete 

 and empirical methods of the laboratory; 

 of general and far-reaching extension of 

 experimental methods in our science. 



But I will return to embryology. It 

 may be doubted whether any period in the 

 long history of this science has been more 

 productive of varied and important dis- 

 coveries than that which followed upon its 

 adoption of experimental methods. In one 

 direction the embryologist went forward 

 to investigations that brought him into inti- 

 mate relations with the physicist, the chem- 

 ist, the pathologist and even the surgeon. 

 A flood of light was thrown on the phenom- 

 ena of development by studies on diiferen- 



