at an ordinary birth.  But do they really believe that at innumerable periods in the earth’s history certain elemental atoms have been commanded suddenly to flash into living tissues?  Do they believe that at each supposed act of creation one individual or many were produced?  Were all the infinitely numerous kinds of animals and plants created as eggs or seed, or as full grown? and in the case of mammals, were they created bearing the false marks of nourishment from the mother’s womb?  Although naturalists very properly demand a full explanation of every difficulty from those who believe in the mutability of species, on their own side they ignore the whole subject of the first appearance of species in what they consider reverent silence.

It may be asked how far I extend the doctrine of the modification of species.  The question is difficult to answer, because the more distinct the forms are which we may consider, by so much the arguments fall away in force.  But some arguments of the greatest weight extend very far.  All the members of whole classes can be connected together by chains of affinities, and all can be classified on the same principle, in groups subordinate to groups.  Fossil remains sometimes tend to fill up very wide intervals between existing orders.  Organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed state; and this in some instances necessarily implies an enormous amount of modification in the descendants.  Throughout whole classes various structures are formed on the same pattern, and at an embryonic age the species closely resemble each other.  Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same class.  I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.  But analogy may be a deceitful guide.  Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction.  We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree.  Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.  When the views entertained in this volume on the origin of species, or when analogous views are generally admitted, we can dimly foresee that there will be a considerable revolution in natural history.  Systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be in essence a species.