THE PALEONTOLOGIC RECORD 473 
THE PALEONTOLOGIC RECORD 
THE CONTINUITY OF DEVELOPMENT 
By Dr. W. D. MATTHEW 
AMERICAN MUSEUM OF NATURAL HISTORY 
ee TINUITY of development in a broad sense hardly calls for 
discussion here. The paleontologic evidence in its favor is s0 
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. his 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 
