Along with this progressive integration there has gone on a progressive differentiation. Vegetal units of whatever order, originally homogeneous, have become heterogeneous while they have become united. Spherical cells aggregating into threads, into laminæ, into masses, and into special tissues, lose their sphericity; and instead of remaining all alike assume innumerable unlikenesses--from uniformity pass into multiformity. Fronds combining to form axes, severally acquire definite differences between their attached ends and their free ends; while they also diverge from one another in their shapes at different parts of the axes they compose. And axes, uniting into aggregates of a still higher order, become contrasted in their sizes, curvatures, and the arrangements of their appendages. Similarly among animals. Those components of them which, with a certain license, we class as morphological units, while losing their minor individualities in the major individualities formed of them, grow definitely unlike as they grow definitely combined. And where the aggregates so produced become, by coalescence, segments of aggregates of a still higher order, they, too, diverge from one another in their shapes. The morphological differentiation which thus goes hand in hand with morphological integration, is clearly what the perpetually-complicating conditions would lead us to anticipate. Every addition of a new unit to an aggregate of such units, must affect the circumstances of the other units in all varieties of ways and degrees, according to their relative positions--must alter the distribution of mechanical strains throughout the mass, must modify the process of nutrition, must affect the relations of neighbouring parts to surrounding diffused actions; that is, must initiate a changed incidence of forces tending ever to produce changed structural arrangements. § 264. This broad statement of the correspondence between the general facts of Morphological Development and the principles of Evolution at large, may be reduced to statements of a much more specific kind. The phenomena of symmetry and unsymmetry and asymmetry, which we have traced out among organic forms, are demonstrably in harmony with those laws of the re-distribution of matter and motion to which Evolution conforms. Besides the myriad-fold illustrations of the instability of the homogeneous, afforded by these aggregates of units of each order, which, at first alike, lapse gradually into unlikeness; and besides the myriad-fold illustrations of the multiplication of effects, which these ever-complicating differentiations exhibit to us; we have also myriad-fold illustrations of the definite equalities and inequalities of structures, produced by definite equalities and inequalities of forces.