Another law of progress, which is quite independent of differentiation, nay, even appears to a certain extent opposed to it, is the law of _centralization_. In general the whole organism is the more perfect the more it is organized as a unit, the more the parts are subordinate to the whole, and the more the functions and their organs are centralized. Thus, for example, the system of blood-vessels is most perfect where a centralized heart exists. In like manner, the dense mass of marrow which forms the spinal cord of vertebrate animals, and the ventral cord of the higher articulated animals, is more perfect than the decentralized chain of ganglia of the lower articulated animals, and the scattered system of ganglia in the molluscs. Considering the difficulty of explaining these complicated laws of progress in detail, I cannot here enter upon a closer discussion of them, and must refer to Bronn’s excellent “Morphologischen Studien,” and to my “General Morphology” (Gen. Morph. i. 370, 550; ii. 257-266). Just as we have become acquainted with phenomena of progress, quite independent of divergence, so we shall, on the other hand, very often meet with divergencies which are not perfecting, but which are rather the contrary, that is retrogressions or degenerations. It is easy to see that the changes which every species of animal and plant experiences cannot always be improvements. But rather many phenomena of differentiation, which are of direct advantage to the organism itself, are yet, in a wider sense, detrimental, inasmuch as they lessen its general capabilities. Frequently a relapse to simpler conditions of life takes place, and by adaptation to them a divergence in a retrograde direction. If, for instance, organisms which have hitherto lived independently accustom themselves to a parasitical life, they thereby degenerate or retrograde. Such animals, which hitherto had possessed a well-developed nervous system and quick organs of sense, as well as the power of moving freely, lose these when they accustom themselves to a parasitical mode of life; they consequently retrograde more or less. There the differentiation viewed by itself is a degeneration, although it is advantageous to the parasitical organism. In the struggle for life such an animal, which has accustomed itself to live at the expense of others, by retaining its eyes and apparatus of motion, which are of no more use to it, would only expend so much material uselessly; and when it loses these organs, then a great quantity of nourishment which was employed for the maintenance of these parts, benefits other parts. In the struggle for life between the different parasites, therefore, those which make least pretensions will have advantage over the others, and this favours their degeneration.