PRESENT CRISIS IN EVOLUTIONARY THOUGHT 3 



gists of the nineteenth century left their imaginations peril- 

 ously unfettered and permitted them to indulge in a veritable 

 orgy of theorizing. Now, however, that the trail blazed by 

 the great Augustinian Abbot, Mendel, has been rediscovered, 

 work of real value is being done with the seed pan, the incu- 

 bator, the microtome, etc., and the wings of irresponsible specu- 

 lation are clipped. Recent advances in this new field of 

 Mendelian genetics have made it possible to subject to critical 

 examination all that formerly went under the name of "ex- 

 perimental evidence" of evolution. Even with respect to the 

 inferential or circumstantial evidence from palaeontology, the 

 enormous deluge of fossils unearthed by the tireless zeal of 

 modern investigators has annihilated, by its sheer complexity, 

 the hasty generalizations and facile simplifications of a 

 generation ago, forcing the adoption of a more critical 

 attitude. Formerly, a graded series of fossil genera 

 sufiSced for the construction of a "palseontological pedi- 

 gree"; now, the worker in this field demands that the chain 

 of descent shall be constructed with species, instead of genera, 

 for links — "Not till we have linked species into lineages, can 

 we group them into genera." (F. A. Bather, Science, Sept. 17, 

 1920, p. 264.) This remarkable progress in scientific studies 

 has tended to precipitate the crisis in evolutionary thought, 

 which we propose to consider in the present chapter. Before 

 doing so, however, it will be of advantage to formulate a clear 

 statement of the problem at issue. 



Evolution, or transformism, as it is more properly called, 

 may be defined as the theory which regards the present species 

 of plants and animals as modified descendants of earlier 

 forms of life. Nowadays, therefore, the principal use of the 

 term evolution is to denote the developmental theory of organic 

 species. It is, however, a word of many senses. In the 

 eighteenth century, for example, it was employed in a sense 

 at variance with the present usage, that is, to designate the 

 non-developmental theory of embryological encasement or 

 preformation as opposed to the developmental theory of epi- 



