196 GENERAL EVOLUTION. 



menced, and tliat 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 ex- 

 istence of rudiments of organs in process of development, while 

 rudiments of organs in process of extinction are results of retarda- 

 tion, occasioned by absolute or complementary loss of growth-force. 

 Many other characters will follow, at a distance, the modification 

 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 requires the light 

 of the sun for its production. The most striking examples 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- evolu- 

 tion be true, these have descended from more highly colored pro- 

 genitors. The flat fishes, also {Pleuronectidce), 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 symmetrical fishes, which were similarly colored 

 on both sides. Now the lower side is colorless, the upper retain- 

 ing often brilliant hues. The influence of sunlight is thus as dis- 

 tinctly discoverable among animals as among plants, where it has 

 been generally accepted as a principle of vegetable physiology.* 



Examples of the Effects of Effort and Use. — a. The Respiratory 

 and Circulatory System of Vertebrates. — It is well known that the 

 succession of classes of Vertebrates is measured first by their adapta- 

 tion to aeration in water, and then by their successive departures 

 from this type in connection with the faculty of breathing air. 

 The same succession of structure is traversed by the embryos of 

 the vertebrates, the number of stages passed being measured by 

 the final status of the adult. This transition takes place in the 

 Batrachia later in development than in any other class. Now, it 

 is well known that the transition or metamorphosis may be delayed 



* In 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 exposure to light. The evolutionist will understand that the 



effect of such influence increases with succeeding generations by the addition to in- 



.herited character of the effect of immediate external cause. 



