The opening of Chapter I
INTRODUCTION--POSITION OF MOLLUSCA IN THE ANIMAL
KINGDOM--CLASSIFICATION--ORIGIN OF LAND AND
FRESH-WATER MOLLUSCAIt is the generally accepted opinion among men of science that all life originated in the sea. Not that all parts of the sea are equally favourable to the development of forms of life. The ocean surface, with its entire absence of shelter or resting-place, and the deep sea, whose abysses are always dark and cold and changeless, offer little encouragement to plant or animal life, as an original starting-point. True, both the surface and the depths of the sea have become colonised by myriads of forms, Mollusca amongst them, but these quarters are in the truest sense colonised, for the ancestors of those who inhabit them in all probability migrated from elsewhere.