Over the parts that are becoming leaf-surfaces, certain of the materialsare metamorphosed into wax. In this place starch passes into one of its isomericequivalents, sugar; and in that place into another of its isomeric equivalents,gum. By secondary change some of the cellulose is modified into wood; whilesome of it is modified into the allied substance which, in large masses,we call cork. And the more numerous compounds thus arising, initiate furtherunlikenesses by mingling in unlike ratios. The yelk, or essential part ofan animal-ovum, having components which are at first evenly diffused amongone another, chemically transforms itself in like manner. Its protein, itsfats, its salts, become dissimilarly proportioned in different localities;and multiplication of isomeric forms leads to further mixtures and combinationsthat constitute minor distinctions of parts. Here a mass darkening by accumulationof hematine, presently dissolves into blood. There fatty and albuminous mattersuniting, compose nerve-tissue. At this spot the nitrogenous substance takeson the character of cartilage; and at that, calcareous salts, gathering togetherin the cartilage, lay the foundation of bone. All these chemical differentiationsslowly become more marked and more numerous.
Simultaneously arise contrasts of minute structure. Distinct tissues takethe place of matter that had previously no recognizable unlikenesses of parts;and each of the tissues first produced undergoes secondary modifications,causing sub-species of tissues. The granular protoplasm of the vegetal germ,equally with that which forms the unfolding point of every shoot, gives originto cells that are at first alike. Some of these, as they grow, flatten andunite by their edges to form the outer layer. Others lengthen, and at thesame time join together in bundles to lay the foundation of woody-fibre.
Before much elongating, certain of these cells show a breaking-up of thelining deposit, which, during elongation, becomes a spiral thread, or a reticulatedframework, or a series of rings; and by the longitudinal union of cells solined, vessels are formed. Meanwhile each of these differentiated tissuesis re-differentiated: instance that constituting the essential part of aleaf, the upper stratum of which is composed of chlorophyll-cells remainingclosely packed, while the lower stratum becomes spongy. Of the same generalcharacter are the transformations undergone by the fertilized ovum, which,at first a cluster of similar cells, quickly reaches a stage marked by dissimilarityof the cells. More frequently recurring fission of the superficial cells,a resulting smaller size of them, and subsequent union of them into an outerlayer, constitute the first differentiation; and the middle area of thislayer is rendered unlike the rest by still more active processes of likekind. By such modifications upon modifications, many and various, arise theclasses and sub-classes of tissues which, intricately combined one with another,compose organs.
Equally conforming to the law are the changes in general shape and inthe shapes of organs. All germs are at first spheres and all limbs are atfirst buds or mere rounded lumps. From this primordial uniformity and simplicity,there take place divergences, both of the wholes and of the leading parts,towards multiformity of contour and towards complexity of contour. Removethe compactly-folded young leaves that terminate every shoot, and the nucleusis found to be a central knob bearing lateral knobs, one of which may growinto either a leaf, a sepal, a petal, a stamen, or a carpel: all these eventually-- unlike parts being at first alike. The shoots themselves also depart fromtheir primitive unity of form; and while each branch becomes more or lessdifferent from the rest, the whole exposed part of the plant becomes differentfrom the imbedded part. So, too, is it with the organs of animals. One ofthe Arthropoda, for instance, has limbs that were originally indistinguishablefrom one another -- composed a homogeneous series; but by continuous divergencesthere have arisen among them unlikenesses of size and form, such as we seein the crab and the lobster. Vertebrate creatures equally exemplify thistruth. The wings and legs of a bird are of similar shapes when they bud-outfrom the sides of the embryo.
Thus in every plant and animal, conspicuous secondary re-distributionsaccompany the primary re-distribution. A first difference between two parts;in each of these parts other differences which presently become as markedas the first; and a like multiplication of differences in geometrical progression,until there is reached that complex combination constituting the adult. Thisis the history of all living things whatsoever. Pursuing an idea which Harveyset afloat, it has been shown by Wolff and Von Baer, that during its developmenteach organism passes from a state of homogeneity to a state of heterogeneity.
