vast range of possibilities. We have then a certain definite type, or group of forms, mathôÙeôÙmatôÙiôÙcally isomorphous, but presenting infinite diversities of outward appearance: which diversities, as Swammerdam {586} said, ex sola nascuntur diversitate gyrationum; and which accordingly are seen to have their origin in differences of rate, or of magnitude, and so to be, essentially, neither more nor less than differences of degree.
In nature, we find these forms presenting themselves with but little relation to the character of the creature by which they are produced. Spiral forms of certain particular kinds are common to Gastropods and to Cephalopods, and to diverse families of each; while outside the class of molluscs altogether, among the Foraminifera and among the worms (as in Spirorbis, Spirographis, and in the Dentalium-like shell of Ditrupa), we again meet with similar and corôÙreôÙsponôÙding forms.
Again, we find the same forms, or forms which (save for external ornament) are mathôÙeôÙmatôÙiôÙcally identical, repeating themselves in all periods of the worldãs geological history; and, irrespective of climate or local conditions, we see them mixed up, one with another, in the depths and on the shores of every sea. It is hard indeed (to my mind) to see where Natural Selection necessarily enters in, or to admit that it has had any share whatsoever in the production of these varied conformations. Unless indeed we use the term Natural Selection in a sense so wide as to deprive it of any purely biological significance; and so recognise as a sort of natural selection whatsoever nexus of causes suffices to differentiate between the likely and the unlikely, the scarce and the frequent, the easy and the hard: and leads accordingly, under the peculiar conditions, limitations and restraints which we call ãordinary circumstances,ã one type of crystal, one form of cloud, one chemical compound, to be of frequent occurrence and another to be rare.