368 RECORDS. 



larger scientific questions that are of enduring interest to all 

 students of nature. It is only fair to point out, however, that a 

 consideration of recent advances in this subject necessarily and 

 speedily leads us into a region that lies remote from everyday 

 experience, surrounded by arid wastes of technical detail, and 

 inhabited by folk who speak an uncouth foreign tongue. With 

 the best of intentions, therefore, the native guide and interpreter 

 has need of some forbearance on the part both of his country- 

 men and of the outlanders whom he attempts to lead. 



I need not dwell on the absorbing, almost tantalizing, interest 

 with which the problem of development has held the attention 

 of naturalists from the earliest times. Twenty centuries and 

 more have passed since Aristotle first endeavored to trace some- 

 thing like a rough outline of its solution. The enormous ad- 

 vances of our knowledge during this long period have taken 

 away nothing of the interest or freshness of the problem ; they 

 have left it, indeed, hardly less mysterious than when the father 

 of science wrote the first treatise on generation. I will not 

 dwell on the epoch-making work of Harvey, Wolff and von 

 Baer, or the curious, almost grotesque controversies of the 

 eighteenth century, when embryology invaded the field of phi- 

 losophy and even of theology. I will only point out that even 

 at that time, when embryology was almost wholly limited to 

 the study of the hen's Qgg, embryologists were already occupied 

 with two fundamental questions, which still remain in their 

 essence without adequate answer, and though metamorphosed 

 by the refinements of more modern observation and experiment 

 still stand in the foreground of scientific discussion. The first 

 of these is the question of preformation 7r;'i"//jr epigenesis — 

 whether the embryo exists preformed or predelineated in the 

 Ggg from the beginning, or whether it is formed anew, step 

 by step, in each generation. The second question is that of 

 mechanism versus vitalism — whether, development is capable 

 of a mechanical or physio-chemical explanation, or whether it 

 involves specific vital factors that are without analogy in the 

 non-living world. It is especially to some modern aspects of 

 these two questions that I invite your attention ; and I shall 



