24 ‘ [Cope. 
1871.] 
or organ used by contact with the objects of its use. This would seem 
to be the remote cause of the deposit of dentine in the used tooth; in the 
thickening epidermis of the hand of the laborer; in the wandering of the 
lymph-cells to the scarified cornea of the frog in Cohnheim’s experiment. 
You cannot rub the sclerotica of the eye without producing an expansion 
of the capillary arteries and corresponding increase in the amount of nu- 
tritive fluid. But the case may be different in the muscles and other 
organs (as the pigment cells of reptiles and fishes) which are under the 
control of the volition of the animal. Here, and in many other instances 
which might be cited, it cannot be asserted that the nutrition of use is 
not under the direct control of the will through the mediation of nerve 
force. Therefore I am disposed to believe that growth force may be, 
through the motive force of the animal, as readily determined to a lo- 
cality where an executive organ does not exist, as to the first segment or 
cell of such an organ already commenced, and that therefore effort is, in 
the order of time, the first factor in acceleration. 
Addition and subtraction of growth force in accordance with the modes 
pointed out below, account for the existence of many characters which 
are not adaptive in their nature, 
Acceleration under the influence of effort accounts for the existence 
of rudiments of organs in’ process of development, while rudiments of 
organs in process of extinction are results of retardation, occasioned by 
absolute or complementary loss of growth force. Many other characters 
will follow, at a distance, the modifications resulting from the operation 
of these laws. 
EXAMPLES OF THE INFLUENCE OF PHysICAL CAUSES. 
This is nowhere better seen than in the case of coloration, which re- 
quires the light of the sun for its production. The most striking ex- 
amples of this are seen in the colorless surface of animals inhabiting the 
recesses of caves, as the blind craw-fish and the Amblyopsis, etc. If evo- 
lution be true, {hese have descended from more highly colored pro- 
genitors. The flat fishes, also (Pleuronectide) as is known, swim on one 
side in adult age, but many of them are hatched symmetrical fishes, or 
nearly so, one eye rotating from one side to the other by a twisting of the 
cranial bones. It is thus probable that they have descended from syin- 
metrical fishes, which were similarly colored on both sides. Now, the 
lower side is colorless, the upper retaining often brilliant hues. The in- 
fluence of sunlight is thus as distinctly discoverable among animals as 
among plants, where it has been generally accepted as a principal of veg- 
etable physiology.* 
EXAMPLES OF THE Errects oF Errort anp Us. 
a The Respiratory and Circulatory System of Vertebrates. It is well 
known that the succession of classes of Vertebrates is measured first by 
*TIn this and similar cases, care must be taken not to misunderstand the writer by supposing 
him to mean that in each generation separately the peculiar coloration is the result of changed ex- 
posure to light. ‘’he evolutionist will understand that the effect of such influence juereases with 
succeeding generations by the addition to inherited character, of the effect of immediate external 
cause, 
