62 SECTIONAL ADDRESSES 



Thus the slits in the neck of the human embryo are compared with the 

 gill clefts of a present-day dogfish, and the similarity between them has 

 been referred to as evidence that the development of the individual, at 

 least in this respect, tends to recapitulate the history of the race. On 

 the other hand the much more striking dissimilarity between the human 

 embryo as a whole and the adult dogfish is given as evidence against the 

 ' law of recapitulation.' What is the sense in which the word ' race ' is 

 being used in these or similar cases ? 



Apart from the point already dealt with, that only collateral ancestors 

 are being compared, the disputants on both sides have in mind the whole 

 sub-kingdom of the vertebrates with a range extending across nearly the 

 whole of geological time, and represented by masses of rocks twenty or 

 more miles in thickness. But when a palaeontologist compares the young 

 stages of growth of Gryphcea incurva with the fully grown Ostrea irregulare, 

 and maintains that the close similarity between them is evidence in favour 

 of the ' principle of recapitulation,' the conception he has of the word 

 ' race ' extends only slightly beyond the bounds of a gens or sequence 

 of closely related species, and corresponds to a range of time represented 

 stratigraphically by only about 30 feet of rocks near the base of the Lias. 

 The Gryphcea material forms practically an unbroken series, almost as 

 continuous as the Great North Road , but the dogfish and human materials 

 are relatively more remote from one. another than are London and 

 Edinburgh. 



In both cases the evidence is valid only as far as it goes and no further. 

 The evidence of the human embryo, relating as it does to the extremities 

 of a sub-kingdom separated by several hundred millions of years, cannot 

 in any way be quoted as invalidating the evidence of Gryphcea concerning 

 the relationship of development to evolution within the limits of a couple 

 of genera, ranging with almost complete continuity across possibly less 

 than a quarter of a million years. In recent years the discussion of these 

 problems has been marked by a strange lack of a sense of proportion, a 

 sense which must be maintained if any progress of thought is to be made. 

 To deny that there is any truth in the principle of recapitulation or, on 

 the other hand, to talk as though it were universally applicable, does not 

 conduce to clear thinking. 



The Evidence of Zaphrentis delanouei. 



Now that some of the confusion in the use of various terms has been 

 cleared up we may proceed to lay a stable foundation for our subsequent 

 thinking by making a detailed analj'sis of a well-established evolutionary 

 series. For this purpose no better example can be taken than that 

 provided by the work of R. G. Carruthers (1910) upon Zaphrentis 

 delanouei. At this point I must express my indebtedness to 

 Mr. Carruthers and to the Director of the Geological Survey for giving 

 me every facility for making a careful re-examination of the salient 

 material upon which this work v/as based. 



This example has the great initial advantage that it nearly fulfils all 

 the requirements of first-class evidence. In the first place it is based 



