1889.] On Variation. 1071 
in the previous conceptions, but does so by assuming with Darwin 
the intermediation of gemmules, a hypothesis to which objection 
has been made in the preceding pages. Brooks's theory also 
fails to admit the origin of variations through mechanical stresses, 
although he seeks for the origin of gemmules through the lack 
of equilibrium between the organization and its environment, 
which embraces that proposition in a less precise form. To 
Weismann we are indebted for the exposition of the separate 
origin and relative isolation of the germ-plasma, but no explan- 
ation of the origin and inheritance of new characters is offered. 
Ryder" has especially dwelt on the physiological division of labor 
seen in the tissues of the organism, and on the special function of 
the germ-plasma as the recipient of impressions through the pro- 
cesses of metabolism ; but he does not go into greater detail 
ON VARIATION: WITH SPECIAL REFERENCE TO 
CERTAIN PALAEOZOIC GENERA. 
BY PROF. JOSEPH F. JAMES. 
T^HE question, " What are the limits of a species ? " has been dis- 
cussed for over two hundred years, and a satisfactory reply to 
it has not yet appeared. None of the numerous answers seem to 
meet all requirements. The conception of what a species really 
is has, too, been greatly modified since the publication of the 
" Origin of Species " by Darwin. It has been assumed by some 
of the more radical naturalists that species as entities are very 
rare, and that their boundaries are so indefinite that practically 
few exist. It is true that certain forms of animals and plants are 
distinct, or possess so few relatives that they seem to stand 
isolated. The Venus's Fly-trap among plants, and the Duck-bill 
among animals, seem to occupy positions which cut them off 
from all other plants or animals ; but such cases are certainly 
exceptional. On the other hand, there are whole groups of 
