ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
29 
or morphogenic stimuli. All external factors which induce qualitatively 
distinct formative processes are morphogenic stimuli. Correlation means 
the induction of specific formative changes (qualitative and quantitative) 
by internal factors. These internal correlations must be distinguished 
from growth-compensations, where one organ exerts an indirect influence 
on the size of another, and from growth-modifications (. Alterations - 
erscheinungen ), where one organ indirectly affects the quality of growth 
in another. 
Under external formative stimuli or the induction of specific forma- 
tive changes by external factors, Herbst treats of the action of light 
(photomorphosis), the action of gravity (baryomorphosis), the action of 
contact (thigmomorphosis), the action of pressures and strains (mechano- 
morphosis), the action of chemical substances (chemomorphosis), the 
influence of habitat in water and air (hydro- and aeromorphosis), and 
the influence of oxygen (oxygenomorphosis). 
After a brief discussion of internal formative stimuli, the author 
passes to a general physiological discussion of what is meant by a specific 
reaction, of the duration and intensity of stimuli, of the dependence of 
the reaction on the developmental stage reached by the organism, of 
the mechanism of reaction, and of the various worth of different stimuli. 
His general summing up is still to follow. 
Hertwig’s Theory of Development.* —Dr. F. v. Wagner has some 
criticisms to offer on Prof. 0. Hertwig’s theory of development. His 
contention is that the real causes of development are internal, in the 
constitution of the fertilised ovum, and that this is the product of 
the ages, an “ TJrsacTiendepot .” What are called external causes are 
conditions, — of normal growth if they are favourable, i. e. if they 
are within the narrow limits for which each ovum is adapted, of 
inhibition or malformation if they are beyond these limits. 
Hertwig speaks of the food of the embryo serving for growth and 
development, of the “ Anlage ” growing and changing continuously at 
the expense of its food ; but Wagner thinks even this innocent statement 
inaccurate, — the nutrition of the embryo, like that of the adult, serves 
for self-preservation. 
To Wagner the causes of development are only of one kind, those 
dependent on the constitution of the germ-cells; Hertwig confuses 
conditions and causes. Regarded causally, development is in all cases 
an “ evolution,” not an “ epigenesis.” It seems to us, however, that 
much of this discussion is only logomachy. 
Dynamical Hypothesis of Inheritance.f — The late Prof. J. A. Ryder 
was a strong opponent of the preformationist doctrine of development, 
and one of his last essays was a forcible criticism of this. Although 
Weismann, with his theory of the continuity of the germ-plasm, is far 
removed from Democritus and his successors down to Darwin, who have 
been supporters of pangenesis doctrine, he is like Democritus in his 
preformationist theory of determinants and biophors. “ To these he 
ascribes powers little short of miraculous, in that he asserts that these 
* Biol. Centralbl., xv. (1895) pp. 777-84, 805-15. 
f Wood’s Holl Biological Lectures, Boston, 1895, pp. vii. and 287 ; 2nd lecture, 
pp. 28-54 (4 figs.). 
