This question, as being one that strikes the imagination, naturally arose even before science possessed the means of settling it, and preceded, in the historical order, that thorough study of individuals on which its solution really depends. When men of science had begun to study living things with other purposes than simply that of deriving from them knowledge that would be available for the medical art, and had gained sufficient information for inductive generalizations, they no longer contented themselves with theories of the origin of groups, but sought to reduce to general principles the structure of living bodies—a thing which previously had been considered only from the topographical point of view, and with reference to what was called the use of the parts; and on these general principles they sought to rest a scientific theory of the origin of natural groups.
A man of keen and powerful intellect, who, had he but lived in our time, would have attained the summit of fame, with marvelous acumen anticipated a doctrine which is steadily tending to become a received scientific theory, viz., that the changes which have occurred in Nature are the effects of constant natural laws. Applying this idea to the natural groups of the animal kingdom, he rejected the hypothesis which ascribed to geological catastrophes the destruction of entire faunæ, and the preparation of the earth's surface for a fresh special creation. The transformation of lower organisms into higher he referred to the action of modifications which, though in themselves inconsiderable, became important from repetition and long accumulation, under the influence of forces whose powers he exaggerated. Species and varieties he regarded as artificial groups. According to him the very simplest organisms are derived, by way of spontaneous generation, from naturally-produced plastic substances; then they mutually diverged by imperceptible differences, so as to constitute a linear series, which, but for the gaps caused here and there by lost species, would present to us the aspect of a continuous system. Under favoring circumstances the organs of an animal are modified; a change in the circumstances causes changes in the structure of the individuals belonging to a species, and is the starting-point for the formation of a new species. Crossing, by producing hybrids, still further multiplies the number of species. And species appear to be fixed, simply because the circumstances appear to be similarly fixed during the brief period embraced in our observations. Transformation is the rule, and in the regular course which it runs we can discover no indications of plan or purpose.
The ideas of Lamarck, being but ill supported by positive proofs, were looked on as mere speculations, plausible but doubtful, or even as dreams, unworthy of science; his generalizations were discredited, and even now, when they reappear, backed by a powerful array of facts, but few ever think of giving due honor to their author.
The attempts made at the same period to form generalizations
