has been partly or mainly brought about by the circumstance that the animals have lived under such conditions as constantly caused them to run as swiftly as possible; which endeavours at swift running caused in the individual hares a development, as in athletes, of the structures that subserve swift running, and that this acquired development, transmitted to the offspring, and accumulated in the course of generations, has been a part or a main cause of the evolution of speed in hares. To take another example; the muscles of a blacksmith's arm are enlarged by exercise. It is contended by the one school that this enlargement, being an acquired not an inborn variation, is not at all transmitted, by the other that it is in part transmitted. To take a third example; suppose a man to be enfeebled by disease. It is contended by the one school that this acquired enfeeblement cannot be transmitted; by the other that it can and is transmitted.
The theory that evolution has resulted from the accumulation of inborn variations alone is usually, but incorrectly, called the theory of evolution by Natural Selection. I say incorrectly, for if acquired variations are transmissible, they must equally with inborn variations be seized upon and accumulated by Natural Selection; and therefore a theory that supposes that the organic world has arisen by the accumulation of acquired variations is as much a theory of evolution by Natural Selection, as a theory that supposes that it has arisen by the accumulation of inborn variations alone; particularly since variations acquired in one generation become, if transmitted, inborn in the next generation.
The theory that acquired traits are transmissible is older than the theory which denies their transmissibility. Lamark, finding evidence of evolution in the organic world, published in the beginning of this century a work in which he attributed all evolution to the accumulation of acquired traits. In 1858 Darwin and
