6o SECTIONAL ADDRESSES 



illustrative material will be confined mainly to well-known examples, 

 descriptions of which are easily accessible. 



Any consideration of the relationship of development to evolution 

 must deal with the subject from two aspects, viz. retrospective and 

 prospective. On the one hand it must inquire whether the evolutionary 

 changes of the past are reflected in development, and if so to what extent. 

 On the other hand it must also inquire whether future evolutionary 

 changes of sudden or of sequential character are foreshadowed in develop- 

 ment. These two aspects are, of course, very closely interwoven with 

 one another in the developmental record, and much confusion, which 

 has crept into discussion in recent years, is due to a want of appreciation 

 of their fundamental distinctness. 



Retrospective Aspect. 



In one form or another the retrospective aspect of the problem of the 

 relationship of development to evolution has attracted the attention of 

 embryologists even in the earliest stages in the growth of their science. 

 This is exemplified by the principles enunciated by Von Baer and Haeckel, 

 even though the former dates back to the pre-evolution age of biology. 

 These two great workers, had they lived to-day, would have been the 

 first to condemn any tendency to make a creed of the form of words in 

 which they expressed the principles they detected ; they would have 

 been the first to welcome any modification or amplification made necessary 

 by the advance of knowledge. No attempt will be made here to trace 

 the history of discussion on these problems, for it has been frequently 

 summarised by various writers referred to in this address. 



Concerning the Use of Terms. 



Some of the disagreement that exists over the questions under dis- 

 cussion is due to diversity in the shades of meaning attached to terms in 

 common use. It will therefore be helpful if some of these are briefly 

 indicated here. 



The term ontogeny is generally taken to mean the development of the 

 individual, but it is not always clear how much of that development the 

 writer has in mind. As long ago as 1909 Cumings pointed out that 

 many biologists used the term ontogeny when they were really visualising 

 only embryogeny. Even Haeckel himself did this. Strictly speaking, it 

 includes all stages of growth from the embryonic through the epembryonic 

 or neanic to the adult. 



With regard to phylogeny, De Beer indicates a common though often 

 quite unconscious restriction in the use of the term when he tells us that 

 ' the distinction between adult and young, i.e. between structures which 

 appear late or early, is drawn principally because it is only the structures' 

 of the adult which are concerned in phylogeny.' Garstang (1921), it 

 seems to me, gives it the correct significance when he defines phylogeny 

 as ' the procession of ontogenies along a given phyletic line of modifica- 

 tion,' though he goes on to point out that ' it is expressed in terms of adult 



