In some cases variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate.

In no case, probably, has time sufficed for the utmost possible amount of development.

In some few cases there has been what we must call retrogression or organisation.
But the main cause lies in the fact that under very simple conditions of life a high organisation would be of no service--possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first step in the advancement or differentiation of parts have arisen?
Mr.Herbert Spencer would probably answer that, as soon as simple unicellular organisms came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any order become differentiated in proportion as their relations to incident forces become different" would come into action.

But as we have no facts to guide us, speculation on the subject is almost useless.

It is, however, an error to suppose that there would be no struggle for existence, and, consequently, no natural selection, until many forms had been produced: variations in a single species inhabiting an isolated station might be beneficial, and thus the whole mass of individuals might be modified, or two distinct forms might arise.