He never was able to elicit the “extensor thrust” in the “spinal dog” by any form of electrical stimula­tion, but only by a particular kind of mechanical stimulus. This peculiarity was also found in the pinna reflex of the cat.

As to the scratch reflex in the dog it was only when it was easily elicitable that it could be evoked by electrical stimula­tion as well as mechanical, and when it was not easily elicitable electrical stimuli failed altogether while mechanical stimuli still evoked it.

He describes the receptor as a mechanism “attuned to respond specially to a certain one or ones of the agencies that act as stimuli to the body,” and points to the fact that electrical stimuli are not of common occurrence in nature and no chance for adapta­tion to evolve in the organism receptors appropriate for such stimuli has been afforded. Such negative facts are at the least suggestive in considering the question of the mode of origin of receptors and end-organs, electrical stimuli being rare in nature.

The subject of the innerva­tion of the skin and its receptors has been treated here in a great measure by the aid of imagina­tion, with some evidence, and a good deal of reconstruc­tion has been attempted, but perhaps this will be pardoned by those who are prepared to carry out a corresponding process with such as Pithecanthropus, Eoanthropus and Saurian monsters from somewhat scanty osseous remains. Any biological theory of the origin of these receptors than the one here put forward is faced with some formidable difficulties, which are probably insurmountable.

CHAPTER XXIV.
THE BUILDING OF REFLEX ARCS.

Assuming the foregoing origin of the innerva­tion of the skin, I submit that between this rudimentary process and the building of sensori-motor arcs in the spinal cord and brain there is a field, almost unlimited, for initiative in the construc­tion of new forms of animal life. The former is nothing without the latter. To leave it without proceeding further is to leave it “in the air” as military writers say. The formation of Receptors, then, both in the skin field and in the higher sense-organs, leads of necessity to the formation, multiplica­tion and co-ordina­tion of reflex arcs. As in an imperfectly organised telephone service after many a repeated stimuli or “rings” the messages begin to reach their destinations, and as by practice the operators better and better learn their business, so the impulses passing through receptors and nerve-fibrils become organized into more or less efficient systems of arcs, and response is secured to them by some effector of gland or muscle. It is not true of man alone that practice makes perfect.

A certain feature of higher animals which distinguishes them from lower must be remembered, and that is that among them the individual becomes increasingly important. Speaking generally, the latter are born and die in large groups, and their lives resemble those of their group more closely than in the former. The struggle of the individual is vividly pictured by Professor Woods Jones in his descrip­tion of the baby of the perfected arboreal animals. He shows how they and the roaming Ungulates and Pelagic Cetacea cannot indulge in large families, and that it is only those forms which have a safe retreat for their young which can avoid reduction of the size of their families, and how the higher apes still more resemble in these respects mankind, as we know it. For the proper study of the “synthesis of the individual” organism this essential fact must be kept in mind.