On the Genesis of Species

by St. George Jackson Mivart

This kind of homology is also explained by Mr. Spencer as the result of the similarity of conditions affecting the two parts. Thus he explains the very general absence of symmetry between the dorsal and ventral surfaces of animals by the different conditions to which these two surfaces are respectively exposed, and in the same way he explains the asymmetry of the flat-fishes (Pleuronectidæ), of snails, &c.

Now, first, as regards Mr. Spencer's explanation of animal forms by means of the influence of external conditions, the following observations may be made. Abundant instances are brought forward by him of admirable adaptation of structure to circumstances, but as to the immense majority of these it is very difficult, if not impossible, to see how external

conditions can have produced, or even tended to have produced them. For example, we may take the migration of one eye of the sole to the other side of its head. What is there here either in the darkness, or the friction, or in any other conceivable external cause, to have produced the first beginning of such an unprecedented displacement of the eye? Mr. Spencer has beautifully illustrated that correlation which all must admit to exist between the forms of organisms and their surrounding external conditions, but by no means proved that the latter are the cause of the former.

Some internal conditions (or in ordinary language some internal power and force) must be conceded to living organisms, otherwise incident forces must act upon them and upon non-living aggregations of matter in the same way and with similar effects.

If the mere presence of these incident forces produces so ready a response in animals and plants, it must be that there are, in their case, conditions disposing and enabling them so to respond, according to the old maxim, Quicquid recipitur, recipitur ad modum recipientis, as the same rays of light which bleach a piece of silk, blacken nitrate of silver. If, therefore, we

attribute the forms of organisms to the action of external conditions, i.e. of incident forces on their modifiable structure, we give but a partial account of the matter, removing a step back, as it were, the action of the internal condition, power, or force which must be conceived as occasioning such ready modifiability. But indeed it is not at all easy to see how the influence of the surface of the ground or any conceivable condition or force can produce the difference which exists between the ventral and dorsal shields of the carapace of a tortoise, or by what differences of merely external causes the ovaries of the two sides of the body can be made equal in a bat and unequal in a bird.

There is, on the other hand, an a priori reason why we should expect to find that the symmetrical forms of all animals are due to internal causes. This reason is the fact that the symmetrical forms of minerals are undoubtedly due to such causes. It is unnecessary here to do more than allude to the beautiful and complex forms presented by inorganic structures. With regard to organisms, however, the wonderful Acanthometræ and the Polycystina may be mentioned as presenting complexities of form which can hardly be thought to be due to other than internal causes. The same may be said of the great group of

Echinoderms, with their amazing variety of component parts. If then internal forces can so build up the most varied structures, they are surely capable of producing the serial, lateral, and vertical symmetries which higher animal forms exhibit. Mr. Spencer is the more bound to admit this, inasmuch as in his doctrine of "physiological units" he maintains that these organic atoms of his have an innate power of building up and evolving the whole and perfect animal from which they were in each case derived. To build up and evolve the various symmetries here spoken of is not one whit more mysterious. Directly to refute Mr. Spencer's assertion, however, would require the bringing forward of examples of organisms which are ill-adapted to their positions, and out of harmony with their surroundings—a difficult task indeed.[^[168]]