I. Haeckel and His Opponents

The thought now lies close at hand, that in Nature also, left to itself, the forms continually vary. And the great power of variation of domesticated organisms does not force us to assume that this property of organic forms is confined within certain limits. We may rather presuppose that in the lapse of vast time-periods a certain form transforms itself into a totally different one, which in its formation diverges from the former to the utmost extent imaginable. The most natural inference then, is this, that the organic species have not arisen independently, each according to a special plan of structure, alongside each other; but that in course of time they have evolved the one from the other. This idea gains support from the views at which Lyell arrived in the history of the earth's development, and which he first published in 1830 in his Principles of Geology. The older geological views, according to which the formation of the earth was supposed to have been accomplished in a series of violent catastrophes, were thereby superseded. Through this doctrine of catastrophes it was sought to explain the results to which the investigation of the earth's solid crust had led. The different strata of the earth's crust, and the fossilised organic creatures contained in them, are of course the vestiges of what once took place on the earth's surface.