I propose to endeavour to follow this ancient precedent, in a manner suited to the limitations of my knowledge and of my capacity.  I shall not presume to attempt a panoramic survey of the world of science, nor even to give a sketch of what is doing in the one great province of biology, with some portions of which my ordinary occupations render me familiar.  But I shall endeavour to put before you the history of the rise and progress of a single biological doctrine; and I shall try to give some notion of the fruits, both intellectual and practical, which we owe, directly or indirectly, to the working out, by seven generations of patient and laborious investigators, of the thought which arose, more than two centuries ago, in the mind of a sagacious and observant Italian naturalist.

It is a matter of everyday experience that it is difficult to prevent many articles of food from becoming covered with mould; that fruit, sound enough to all appearance, often contains grubs at the core; that meat, left to itself in the air, is apt to putrefy and swarm with maggots.  Even ordinary water, if allowed to stand in an open vessel, sooner or later becomes turbid and full of living matter.

The philosophers of antiquity, interrogated as to the cause of these phenomena, were provided with a ready and a plausible answer.  It did not enter their minds even to doubt that these low forms of life were generated in the matters in which they made their appearance.  Lucretius, who had drunk deeper of the scientific spirit than any poet of ancient or modern times except Goethe, intends to speak as a philosopher, rather than as a poet, when he writes that “with good reason the earth has gotten the name of mother, since all things are produced out of the earth.  And many living creatures, even now, spring out of the earth, taking form by the rains and the heat of the sun."[1] The axiom of ancient science, “that the corruption of one thing is the birth of another,” had its popular embodiment in the notion that a seed dies before the young plant springs from it; a belief so widespread and so fixed, that Saint Paul appeals to it in one of the most splendid outbursts of his fervid eloquence:—­

“Thou fool, that which thou sowest is not quickened, except it die."[2]

[Footnote 1:  It is thus that Mr. Munro renders

“Linquitur, ut merito maternum nomen adepta
Terra sit, e terra quoniam sunt cuncta creata. 
Multaque nunc etiam exsistant animalia terris
Imbribus et calido solis concreta vapore.”

De Rerum Natura, lib. v. 793-796.

But would not the meaning of the last line be better rendered “Developed in rain-water and in the warm vapours raised by the sun"?]

[Footnote 2:  1 Corinthians xv. 36.]

The proposition that life may, and does, proceed from that which has no life, then, was held alike by the philosophers, the poets, and the people, of the most enlightened nations, eighteen hundred years ago; and it remained the accepted doctrine of learned and unlearned Europe, through the Middle Ages, down even to the seventeenth century.