372 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
a differentiated form of protoplasm has gradually produced changes 
in the structural arrangement of its molecules, whereby they have 
effectively responded to the exigencies of their environment. Struc- 
tural changes thus effected have a tendency to become hereditary 
qualities. To this extent light may be said to have evolved the 
structures which enter into the formation of the eyes of the various 
classes of animals. Herbert Spencer, when referring to molecular 
changes of the kind to which we have referred, states that there go 
on in all organisms certain changes of structure and functions that 
are directly consequent on changes in the incident forces, inner 
changes by which outer changes are balanced, and the equilibrium 
restored—rearrangements which produce an exactly counterbalanc- 
ing force.t But the result, though as a rule progressive under the 
laws of natural selection, is not always so, as for instance in the case 
of internal parasites which lack even’a digestive tract, because a 
stomach is unnecessary in an animal which lives bathed in the 
nutrient fluids of its host. 
The other hypothesis which we mentioned as, at present, engaging 
the attention of biologists to enable them to explain the origin of 
new species of animals and plants, is the theory which has been 
advocated by the well-known Dutch botanist, Prof. Hugo de Vries. 
De Vries, in the preface to his work ‘‘Species and Varieties, their 
Origin by Mutation,” observes that the current belief assumes that 
species are slowly changed into new types. In contradiction to this 
conception, the theory of mutation assumes that new species and vari- 
eties are produced from existing forms by sudden leaps. The parent 
type itself remains unchanged throughout this process, and may 
repeatedly give birth to new forms. These may arise simultaneously, 
and in groups, or separately at more or less widely distant periods. 
De Vries lays stress on the difference between slight structural 
changes:in plants, and mutations; the former he holds are subject to 
fluctuations and occur continually from one to other generations. 
Mutations, on the other hand, are rare and occur intermittently; 
they do not show any ascending law of frequency. Fluctuations do 
not lead to a permanent change in the increase of the species unless 
there be very rigorous selection, and even then, if the selection be 
slackened, there is regression to the old mean; mutations lead per 
saltum to a new specific position, and there is no regression to the 
old mean. De Vries maintains that fluctuations do not yield any- 
thing really new, they imply a little more or less of characters already 
present; mutations are novelties, they imply some new pattern, 
some new position of organic equilibrium. De Vries holds that no new 
species can be established without mutation. ‘‘When a mutation has 
1 Principles of Biology, by H. Spencer, vol. 1, pp. 484, 442. 
