What, then, is the place of Agassiz in biological science? What is the relation of Agassiz to Darwin--of Agassizian development to Darwinian evolution? I answer, it is the relation of formal science to physical or causal science. Agassiz advanced biology to the _formal_ stage; Darwin carried it forward, to some extent at least, to the _physical_ stage. All true inductive sciences in their complete development pass through these two stages. Science in the one stage treats of the _laws_ of phenomena; in the other, of the _causes_ or explanation of these laws. The former must precede the latter, and form its foundation; the latter must follow the former, and constitute its completion. The change from the one to the other is always attended with prodigious impulse to science. To illustrate: Until Kepler, astronomy was little more than an accumulation of disconnected facts concerning celestial motions--abundant materials, but no science; piles of brick and stone, but no building. Kepler reduced this chaos to beautiful order and musical harmony by the discovery of the three great laws which bear his name, and therefore he has been justly called the legislator of the heavens--_the lawgiver of space_. But, had he been asked the _cause_ of these beautiful laws, he could only have answered, “The _first cause_--the direct will of the Deity.” A good answer and a true, but not scientific; because it places the question beyond the domain of science, which deals only with second or physical causes. But Newton comes forward and gives a _physical cause_. He shows that all these beautiful laws are the necessary result of gravitation; and thus astronomy becomes a physical science. So, until Agassiz, the facts of geological succession of organic forms were in a state of lawless confusion. Agassiz by establishing the three great laws of succession, which ought to bear his name, reduced this chaos to order and beauty; and, therefore, he might justly be called the legislator of geological history--the _lawgiver of time_. But, when asked the cause of these laws, he could only answer, and did indeed answer, “The plans of the Creator.” A noble answer and true, but not scientific. Darwin now comes forward and gives, partly at least, the cause of these laws. He shows that all these beautiful laws are explained by the doctrine of “origin of species by derivation with modifications”; that these laws are not ultimate, but derivative from more fundamental laws of life; and thus biology is advanced one step, at least, toward the causal stage. Newton and Darwin substituted second causes for first cause--natural for supernatural. They each in his own department broke the bonds of supernaturalism in the domain of Nature.