Unity of membership. How secured

Now of the nature of this last kind of unity, the most important whether in moral or in those material things with which we are at present concerned,there is this necessary to be observed, that it cannot exist between things similar to each other. Two or more equal and like things cannot be members one of another, nor can they form one, or a whole thing. Two they must remain, both in nature and in our conception, so long as they remain alike, unless they are united by a third different from both. Thus the arms, which are like each other, remain two arms in our conception. They could not be united by a third arm, they must be united by something which is not an arm, and which, imperfect without them as they without it, shall form one perfect body; nor is unity even thus accomplished, without a difference and opposition of direction in the setting on of the like members. Therefore among all things which are to have unity of membership one with another, there must be difference or variety; and though it is possible that many like things may be made members of one body, yet it is remarkable that this structure appears characteristic of the lower creatures, rather than the higher, as the many legs of the caterpillar, and the many arms and suckers of the radiata, and that, as we rise in order of being, the number of similar members becomes less, and their structure commonly seems based on the principle of the unity of two things by a third, as Plato has it in the Timæus, § II. abstract oil painting