Indeed,his whole theory of induction implies the possibility of reasoning from one property or attribute to another.Make a change in one and the other must be changed.He sees this clearly in the case of organised bodies.(81)In that case,he says,there is a 'presumption'that the properties are 'derivable'and therefore 'caused,'because there we have sequences or one process following another.He thus seems to limit his 'natural kinds'chiefly to chemical compounds.There we have properties lying side by side and not 'derivable,'that is,not to be inferred by us from the properties of the elementary constituents.The very attempt to derive them is idle.As any event may cause any other,however unlike,so any set of properties may be simply stuck together.Bacon is again reproved(82)for assuming that 'every object has an invariable coexistent.'The ultimate properties,so far as we can conjecture,are 'inherently properties of many different kinds of things,not allied in any other respect.'They simply lie side by side,without reference to each other.Thus Mill pushes his empiricism to assuming not only that our knowledge of properties must rest upon direct observation,but that there is absolutely no connection or 'cause'to be known.The 'kind'after all,which was meant to be an essential bond,turns out to be itself a purely arbitrary collection of attributes,and we have to ask whether it does not lose all the significance which he attached to it.The 'collocation'means that the attributes simply lie side by side,and yet are always conjoined.The tie which combines them is undiscoverable,and therefore for us non-existent.It is,as he rightly insists,important that our classification should correspond to natural kinds.'Kinds,'he says,are classes 'between which there is an impassable barrier';the logical class is arbitrary,but the real class is an essential fact,His illustration is remarkable.He holds that the 'species of plants are not only real kinds,but are probably all of them real lowest kinds,infimae species,'and that further subdivision would lead to no valuable results.(83)The doctrine that the species of botany must correspond to 'real kinds 'is curious in a writer who was himself a botanist and familiar with the difficulty of making absolute divisions between kinds.The conflict with the conceptions implied in Darwinism is of the highest importance.(84)The distinction between 'kinds,'according to Darwin,is not absolute,for it is the product of gradual divergence from a single form.But,on the other hand,the kinds existing at a given time are discrete.There are gaps between them,as Mill remarks;though,in so far as they have a common origin,not absolutely insuperable gaps.This implies that the organism does not correspond to a mere aggregate of disconnected attributes,so that the difference of kinds would be simply a difference of more or less,and each type pass into the other by imperceptible gradations.We are obliged to suppose a system of reciprocal relations,so that any change in one organ involves correlated changes in others;and thus species diverge along different lines instead of remaining constant or simply adding on new properties.Mill,it seems,has to admit of kinds in order to account for the possibility of inference;but then,as he wishes to avoid 'mystical bonds,'and inferences from 'definitions,'and the scholastic beggings of the question,he declares the relation between the attributes to be 'accidental'or 'uncaused.'Hence,though he sees the difficulty and recognises the probability of 'causation'in organised bodies,he really reduces the 'kind'to be a mere aggregate,and destroys the very organic bond of which he is in need.
VII.UNIFORMITY OF NATURE
