PROGRESS IN ZOOLOGY — WILSON. 399 



US in the early nineties with Driesch's memorable discovery that by 

 a simple mechanical operation we can at will cause one egg to pro- 

 duce 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 

 feeling of shock w^e took the measure of our ignorance and saw the 

 whole problem of development reopened. 



The immediate and most important result of this Avas to stimulate 

 a great number of important objective investigations in embryologj^ 

 But let me pause for a moment to point out that at nearly the same 

 time a similar reawakening of interest in the experimental investi- 

 gation 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 animals and their reactions to stimuli ; on heredity, varia- 

 tion 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 conventional boundary lines; of revolt from 

 speculative systems toward the concrete and empirical methods of 

 the laboratory; of general and far-reaching extension of experi- 

 mental 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 discoveries than that which followed upon 

 its adoption of experimental methods. In one direction the embry- 

 ologist went forward to investigations that brought him into inti- 

 mate relations with the physicist, the chemist, the pathologist, and 

 even the surgeon. A flood of light Avas thrown on the phenomena 

 of development by studies on differentiation, regeneration, trans- 

 plantation, and grafting; on the development of isolated blastomeres 

 and of egg fragments; on the symmetry and polarity of the egg; on 

 the relations of development to mechanical, physical, and chemical 

 conditions in the environment; on isolated living cells and tissues, 

 cultivated like microorganisms, outside the body in vitro ; on fertili- 

 zation, artificial parthenogenesis, and the chemical physiology of 

 development. In respect to the extension of our real knowledge these 

 advances constitute an epoch-making gain to biological science. 

 And 3'et these same researches afford a most interesting demonstra- 

 tion of how the remoter problems of science, like distant mountain 

 peaks, seem to recede before us even while our actual laiowledge is 

 rapidly advancing. Thirty years after Eoux's pioneer researches we- 

 find ourselves constrained to admit that in spite of all that we have 

 learned of development the egg has not yet yielded up its inmost 

 secrets. I have referred to the admirable discovery of Driesch con- 

 cerning the artificial production of twins. That brilliant leader of 



