Sir,--Against your correspondent’s preference for vernacular terms may be set a remark by Dr. Whewell, that ‘the loose and _infantine_ grasp of common language cannot hold objects steadily enough for scientific examination, or lift them from one stage of generalization to another. They must be secured by the rigid mechanism of a scientific phraseology.’ To say that ‘the first vertebrate must have been the product of innumerable antecedent factors,’ is, perhaps, an expression more puzzling than if one merely said that ‘its immediate progenitors’ were ‘a single pair;’ but then it has the advantage of being considerably more suggestive, and a good deal more to the purpose. Among these ‘factors’ of a living creature parents commonly find a place,--two parents generally; in some cases, only one; in some cases, not even one, if we are to believe the advocates of spontaneous generation, or the still popular view in regard to the Creation. To the parental factors of any particular offspring must be added those which are constituted by food and climate and a multitude of changes and chances in the whole course of its existence. Some characters it may derive from remote ancestors, transmitted through, though not developed in, its ‘immediate progenitors.’ It is now forty years since two French anatomists showed the possibility, or even probability, of a connection between the molluscous cuttle-fish and the vertebrate type; and, considering the propensity of the highest vertebrate for perpetually squirting ink at those who meddle with it, most readers of controversy will think the instance well selected. Those who wish to be told what the first vertebrate was probably like, should first accurately define what they mean by a vertebrate. Is a jointed backbone the only essential? And if so, how many vertebræ are essentially requisite? A creature with two vertebræ would be as much a vertebrate as a creature with three. What shall we say, then, of a creature possessing, if we may use the expression, but a single vertebra? It would not have a jointed backbone. It would not be a vertebrate. Yet, by the simple multiplication of similar parts, in accordance with a thousand analogies, a vertebrate could be developed from it. By the differentiation of these similar parts, which not only might follow, but must follow, a great variety of species could easily be evolved. Those who really take an interest in the problems connected with this subject, and whose ‘convictions’ are not too strong and absolute to be swayed by ascertained facts and logical reasoning will do well to study the ‘Principles of Biology,’ Herbert Spencer. They will there see by what extraordinarily simple gradations the lowliest organisms are connected with higher and the highest forms. They will there find that ‘modification of characters’ is a doctrine less intricate than might be supposed, even if it cannot be wholly explained in words of one syllable.