1889.] On Variation. 108 1 
We strongly suspect that a considerable amount of hesitation 
still exists among palaeontologists against the acceptance of the 
idea of a former wide extension of species, both in time and in 
area. But it would appear that many are beginning to realize 
that variability must have existed in the past as well as at pres- 
ent ; and that many species may have lived through several of 
the hard and fast periods into which geologists have divided 
geologic time. We have, for example, the testimony of Prof 
James Hall, who {rhnerican Geologist, Feb., 1890, p. 122), in 
remarks made before the Geological Society of America, refers 
to the great variability of Spirifera and its allies. So, too, the 
large genus Orthis shows great variation in some of the species, 
as does also Endoceras. Mr. Matthew has recognized great vari- 
ability in certain species of Paradoxides, and Mr. C. D. Wolcott 
has noted a remarkable series of variations in Plenelliis gilberti, 
especially in the features of the head (Bulletin U. S. Geol. Sun, 
No. 30, pp. 173-180). These variations he considers to be the 
result of the retention, by some individuals, of certain embr>'onic 
features which are lost by the generality of the specimens after 
they attain a certain definite size. 
There has developed, of late, a tendency to split some of the 
eariier and larger Palaeozoic groups up into numerous genera, 
which are separated by few definite characters. It is especially 
prevalent among the lower orders, the corals and polyzoa, though 
the tendency is not confined here. Among many of the earlier 
forms it is extremely difficult to decide on any limitations to genera. 
If we take external characters as a guide, one set of writers say, 
" Those are of no value : use the internal structure." But when 
we turn to this, another set tell us, " Those features are uttedy 
worthless : use the external form." Examination proves that 
certain species having a similar outward form have a different 
external structure ; and certain others with quite diverse aspects 
have a like internal structure. The fact seems to be that in many 
of these early forms generic characters are not settled. The 
structure has not become stable enough to present definite feat- 
ures, and so many of the attempts to formulate definitions for 
genera are set at naught. Let us turn our attention now to some 
