1880. | A Review of the Modern Doctrine of Evolution. 177 
of no sufficient use. They are all due either to excess or defect 
of growth force; they are either consequences of a removal of 
nutritive material to other portions of the body; or they are due 
to an excess of such material which renders an organ or part use- 
less through disproportionate size. Of the former class may be ` 
cited the absence of the tail in some monkeys and birds; also of © 
the teeth in some Cetaceans; of the latter kind are the enormous 
tusks of the mammoth and the recurved superior canines of the 
babyrussa. The change of destination of this material has been 
probably due to the construction of adaptive machines whose 
perfection from time to time has required the use of larger and 
larger proportions of force and material. 
In considering the origin of adaptive structures, two alternative 
propositions are presented to us. Did the occasion for its use 
follow the appearance of the structure, or did the need for the 
structure precede its appearance? The following answer to the 
question has always been the most intelligible to me. Animals 
and plants are dependent for existence on their environment. It 
is an every-day experience that changes in environment occur 
Without any preparation for them on the part of living things. If 
the changes are very great, death is the result. It is evident that 
the influence of environment is brought to bear on life as it is, or 
has been, and that special adaptations to it on their part must fol- 
low, not precede changes of climate, topography, population, etc. — 
We have another important consideration to add to this one,-viz: 
the well-known influence of use, 7. e., motion, on nutrition. Exer- 
cise of an organ determines nutritive material to it, and the ner- 
vous or other influence which does this, equally determines 
nutritive material to localities in the body to which an effort to 
Move is directed, whether an executive organ exist there or not. — 
The habit of effort òr use determining the nutritive habit must be 
inherited, and result in the growing young, in additional strut 
ture. Change of structure, denied to the adult on account of its 
fixity, will be realized in the growing or plastic condition of foetal 
or infant life. The two considerations here brought forward lead 
me to think that the cause of acceleration, in many adaptive 
Structures, is environment alone, or environment producing move- — 
ments, which in turn modify structure. The character of the a 
stimulus in the successive grades of life may be expressed by the — A 
‘ollowing table, passing from the lowest to the highest: : 
