==Evolution==—­A second conception, whose influence upon-the development of biology was even greater, was the doctrine of evolution.  It is true that the doctrine of evolution was no new doctrine with the middle of this century, for it had been conceived somewhat vaguely before.  But until historical geology had been formulated, and until the idea of the unity of nature had dawned upon the minds of scientists, the doctrine of evolution had little significance.  It made little difference in our philosophy whether the living organisms were regarded as independent creations or as descended from each other, so long as they were looked upon as a distinct realm of nature without connection with the rest of nature’s activity.  If they are distinct from the rest of nature, and therefore require a distinct origin, it makes little difference whether we looked upon that origin as a single originating point or as thousands of independent creations.  But so soon as it appeared that the present condition of the earth’s crust was formed by the action of forces still in existence, and so soon as it appeared that the forces outside of living forces, including astronomical, physical and chemical forces, are all correlated with each other as parts of the same store of energy, then the problem of the origin of living things assumed a new meaning.  Living things became then a part of nature, and demanded to be included in the same general category.  The reign of law, which was claiming that all nature’s phenomena are the result of natural rather than supernatural powers, demanded some explanation of the origin of living things.  Consequently, when Darwin pointed out a possible way in which living phenomena could thus be included in the realm of natural law, science was ready and anxious to receive his explanation.

==Cytology.==—­A third conception which contributed to the formulation of modern biology was derived from the facts discovered in connection with the organic cell and protoplasm.  The significance of these facts we shall notice later, but here we may simply state that these discoveries offered to students simplicity in the place of complexity.  The doctrine of cells and protoplasm appeared to offer to biologists no longer the complicated problems which were associated with animals and plants, but the same problems stripped of all side issues and reduced to their lowest terms.  This simplifying of the problems proved to be an extraordinary stimulus to the students who were trying to find some way of understanding life.

==New Aspects of Biology==.—­These three conceptions seized hold of the scientific world at periods not very distant from each other, and their influence upon the study of living nature was immediate and extraordinary.  Living things now came to be looked upon not simply as objects to be catalogued, but as objects which had a history, and a history which was of interest not merely in itself, but as a part of a general plan.  They were no longer