76 THE PALEONTOLOGIC RECORD 



THE CONTINUITY OF DEVELOPMENT 



BY DB. W. D. MATTHEW 



AMERICAN MUSEUM OP NATURAL HISTORY 



/CONTINUITY of development in a broad sense hardly calls for 

 \-# discussion here. The paleontologic evidence in its favor is so 

 extensive and so universal that the perfection of the proof is merely a 

 question of the completeness of the evidence. The question for dis- 

 cussion is rather as to the method of race development and specific 

 change whether continuous, by the slow accumulation of minute in- 

 dividual variations, definite or indefinite, through the influence of 

 natural selection or of other causes or discontinuous by the sudden 

 appearance of distinct mutations or sports, usually of subspecific or 

 specific value, sometimes of generic value. This question is much de- 

 bated nowadays, and it would seem that the evidence from paleontology 

 ought to be of the first importance in deciding it. 



It is very commonly asserted that this evidence is strongly in favor 

 of discontinuous development. This would mean that new species and 

 even genera appear, as a rule, suddenly at certain levels, and that the 

 record of a phylum is not usually a slow continuous change from one 

 species into another as we pass upward from stratum to stratum; but 

 that one species has a certain vertical range and is then supplanted by 

 another species, this in turn by a third, and so on, each successive 

 stage being an advance over the preceding, but the species overlapping 

 instead of grading. 



I think that there is no question but that in vertebrate paleontology 

 the evidence taken at its face value does appear to be very distinctly 

 in favor of discontinuous development. Where we are able to follow a 

 phylum of Tertiary mammalia through a series of strata in one locality, 

 we find that the successive stages appear, as a rule, full formed at cer- 

 tain levels, supplant and replace the more primitive stages, and are in 

 turn supplanted and replaced by more advanced stages. In former 

 years, when the records of locality and level were less exact, it was 

 possible to arrange a series of gradations from one stage to another 

 among the specimens pertaining to a particular phylum, and to assume 

 that this gradation corresponded to the levels in the formation at 

 which the specimens had been collected, and that the specific change 

 was through continuous gradation. The more exact records of locality 

 and level and the more extensive and complete collections in recent 

 years have in general failed to confirm this arrangement. In the great 

 majority of cases, so far as the record shows, new species appear already 



