Man passes freely from one aggregation to another. He can change his nation or his city; he can belong to a dozen organizations--biologically speaking, can be aggregated in a dozen different ways--and play a different part as unit in each. He can follow one profession in the morning, another at night, and be a hobby-horse rider in between. This plastic mind has endowed him with a new biological possibility. He can do what no other organism can--he can be both specialized and generalized at one and the same time. In biology, the aggregation of units to form units of higher grade has been always followed by division of labour among the units; and this division of labour has, in all infrahuman history, been made possible only by an irreversible specialization.[11] A soldier-ant is a soldier, and there its possibilities end. It cannot do what the worker or the queen can do. A muscle-cell, because it has gained the power to contract, is cut off from other possibilities; it cannot secrete, or digest, or carry messages. The aggregate of nerve-cells which makes the physical basis of mind is held fixed to its post, incapable of turning to other functions. It follows that the units of all such aggregates are subordinate to the whole--they have lost their independence, and can often no longer be considered as individuals at all, except historically. But in man, none of these things hold. A man can for half his day be the merest cog, subordinate in every detail of his action to the needs of the community, but for the other half be himself, a full and complete individuality, making the community serve his own ends and needs. For him, aggregation does not mean complete and irreversible subordination; his specialization is reversible, and indeed his potentialities as an individual actually increase with the increased individuality of the aggregate to which he belongs. Bearing these differences in mind, we may turn to consider how our doctrine of progress helps us in studying humanity. At the outset we must guard ourselves against the idea that human society has reached any high level of biological individuation. I may perhaps quote from what I have written elsewhere: “If we were to draw a parallel between primitive types of society and some primitive mammal such as a duck-billed platypus, and to compare the course which we hope society will in time accomplish with what has been accomplished in the progress of the mammalian type from a creature resembling the platypus up to man, with what creature should we have to compare the existing state of human communities? I venture to say that we should be flattering ourselves if we were to fix upon the dog.”