e from the study of this form alone; but the ostrean form is the
base of a series, from the summit of which we get a clearer view."
(_Amer. Nat._, pp. 18-20.)

Here we see, plainly brought out by Jackson's researches, that the
Lamarckian factors of change of environment and consequently of habit,
effort, use and disuse, or mechanical strains resulting in the
modifications of some, and even the appearance of new organs, as the
adductor muscles, have originated new characters which are peculiar to
the class, and thus a new class has been originated. The mollusca,
indeed, show to an unusual extent the influence of a change in
environment and of use and disuse in the formation of classes.

Lang's treatment, in his _Text-book of Comparative Anatomy_ (1888), of
the subjects of the musculature of worms and crustacea, and of the
mechanism of the motion of the segmented body in the Arthropoda, is of
much value in relation to the mechanical genesis of the body segments
and limbs of the members of this type. Dr. B. Sharp has also discussed
the same subject (_American Naturalist_, 1893, p. 89), also Graber in
his works, while the present writer in his _Text-book of Entomology_
(1898) has attempted to treat of the mechanical origin of the segments
of insects, and of the limbs and their jointed structure, along the
lines laid down by Herbert Spencer, Lang, Sharp, and Graber.

W. Roux[263] has inquired how natural selection could have determined
the special orientation of the sheets of spongy tissue of bone. He
contends that the selection of accidental variation could not originate
species, because such variations are isolated, and because, to
constitute a real advantage, they should rest on several characters
taken together. His example is the transformation of aquatic into
terrestrial animals.

G. Pfeffer[264] opposes the efficacy of natural selection, as do C.
Emery[265] and O. Hertwig. The essence of Hertwig's _The Biological
Problem of To-day_ (1894) is that "in obedience to different external
influences the same rudiments may give rise to different adult
structures" (p. 128). Delage, in his _Theories sur l'Heredite_,
summarizes under seven heads the objections of these distinguished
biologists. Species arise, he says, from general variations, due to
change in the conditions of life, such as food, climate, use and disuse,
very rarely individual variations, such as sports or aberrations, which
are more or less the result o