“If the broad lines do not hold, then we must sink into irrationality or turn to flagrant supernaturalism.”
Let us then remind ourselves of some of these broad lines.
We know that there was a time when the earth, hot and fiery, could not have been the abode of life. Of the first origins of life we know nothing and guess little. What we can justifiably surmise is that the protoplasm of the original organisms was not yet differentiated into cytoplasm and nucleus, and that sexuality had not yet arisen. The bacteria, however specialized in other ways, are still in this primitive condition.
Later, we can with great probability infer that the independent units into which the stuff of life was subdivided reached a size which, though still minute, was at least not beyond or even close to the limits of microscopic vision; they were further provided with a nucleus, and occasionally underwent sexual fusion. In other words, they showed an organization which we call cellular; they were free-living cells. Such unicellular creatures must have been at one epoch sole inhabitants of the earth, and diverged into the most manifold types of structure and modes of life. Such of them as led an animal as opposed to a plant type of existence would be classified under the Protozoa or unicellular animals.[5]
The colonial habit gives advantages of increased size and greater rapidity of motion, of which many Protozoa have availed themselves. A colonial existence once attained, division of labour, at first between the germinal and the somatic, later between different types of somatic units, will be a further advantage. Such organisms, of which we cannot say definitely whether they are compound aggregates or single wholes, would represent the most natural link between the unicellular Protozoan and the rest of the animal kingdom, the multicellular forms or Metazoa. And indeed such organisms exist at the present day—organisms such as Volvox, Zoothamnium, Proterospongia, and Myxidium—as adjuvant and confirmatory of our reasonable faith.
The multicellular organisms appear to have originated twice over, by divergent routes. There are the true Metazoa, to which belong all the higher types, and the Parazoa or sponges, which have never passed beyond a very primitive type of structure. Both start as simple sacs, whose walls are formed from two primary sheets or layers of cells. Leaving sponges out of account, the Hydroid polyps are the simplest representative of this grade of structure, while some of the Jelly-fish and Siphonophores have attained the utmost limit of its inherent possibilities.
The next great step was the intercalation of a third primary layer between the other two. The result of this, the so-called triploblastic type of organization, gives the ground-plan for all subsequent organizations; and later evolution consists mainly in the evolution of this ground-plan.
In other words, we can now pass from the consideration of the general plan of life’s architecture to that of its details. During the next great tract of time, that which was novel in life (for we must not be guilty of a petitio principii in yet speaking of “advance” or “progress”) was brought about in two main ways—by an increase in the size of organisms, and by an increase in the efficiency of their working.
The simplest Metazoa, such as the polyps, as well as the simplest three-layered forms, such as the free-living flat-worms, are all small, composed of an amount of material comparable with that contained in a single one of our hairs. In every group of Metazoa, increase of size is one of the main features that accompanies specialization, and the more specialized groups possess a higher average size than the less.