82 SECTIONAL ADDRESSES. 



advance the former adult is succeeded by a new one, and slips back into 

 the ontogeny as a developmental stage. Let me briefly state why I am 

 unable any longer to accept this theory. Firstly, it assumes that new 

 steps in evolution are first manifested at the end of the ontogeny, i.e. in 

 the ordinary course of adult life. I can find little or no evidence which 

 supports this proposition, and an overwhelming mass of evidence which 

 points against it. An example or two in Mollusca will be brought before 

 you for consideration. Yet this assumption has even been used to support 

 the theory of the inheritance of functional modifications acquired during 

 the active life. Secondly, it is inconsistent with the actual course of 

 development, which often preserves ancestral modes of development of 

 individual organs, but as often as not introduces difierent organs at periods 

 independent of any probable phyletic time-scale. The totality of an 

 ontogenetic stage is thus normally different from the tout ensemble of any 

 ancestor. Thirdly, it ignores what I regard as the chief outcome of 

 modern Genetics. When this subject was last discussed in Section D, 

 Mendel's principles had not been heard of, and Galton's Law of Ancestral 

 Inheritance was the only generalisation in the field. There was nothing 

 then to prevent us from assuming, and much to persuade us, that some- 

 how or other the successive stages of growth were the expression of 

 successive inheritances. To-day, on the other hand, such a phrase seems 

 an anachronism. I feel bound to assume that development is the expres- 

 sion of a single inheritance. I take it that, whatever I may think as to 

 the resemblance between this ontogenetic stage and that extinct ancestor, 

 I may not assume any inheritance of the ancestral stage itself. My boy 

 may be like his maternal great-grandfather and his sister like her paternal 

 grandmother, but, as the phylogeny has been the same, the ancestral 

 stages as such have obviously not been inherited ; and we now know why, 

 or rather how, that comes about. 



Viewing development then as the sequential expression of a single 

 inheritance, Science confirms Wordsworth's observation of more than a 

 century ago (1802) that 



' The Child is father of the Man,' 



and, subject always to the influence of environing conditions, our stages 

 of development are ' bound each to each ' by a necessitarian chain of 

 progressive differentiations, each stage depending on its predecessor 

 and determining its successor. The bearings of this doctrine on the 

 problem before us do not appear as yet to have been fully appreciated, 

 but squarely faced, they present issues which are of fundamental 

 importance. 



We have seen in the life-histories of Dentalium and Yoldia that a 

 particular larval organ, the prototroch, can undergo considerable adaptive 

 changes with great advantage to the race, and after serving its purpose 

 can be absorbed, if small, or cast aside, if large, without leaving even a 

 scar. You will note that the unity of the inheritance, and the necessitarian 

 sequence, are not broken by this phenomenon. The prototroch is not a 

 preliminary stage in the formation of any adult organ. If you regard the 

 adult as the final complex resulting from a number of differentiating cell- 

 lineages, the prototroch is only a little subsidiary twig near the base, on 



