Each nerve cell receives connections from other nerve cells at six sites called synapses. But here is an astonishing fact—there are about one million billion connections in the cortical sheet. If you were to count them, one connection (or synapse) per second, you would finish counting some thirty-two million years after you began. Another way of getting a feeling for the numbers of connections in this extraordinary structure is to consider that a large match-head's worth of your brain contains about a billion connections. Notice that I only mention counting connections. If we consider how connections might be variously combined, the number would be hyperastronomical—on the order of ten followed by millions of zeros. (There are about ten followed by eighty zero's worth of positively charged particles in the whole known universe!)

I am delighted that I have found a new reaction to demonstrate even to the blind the structure of the interstitial stroma of the cerebral cortex. I let the silver nitrate react with pieces of brain hardened in potassium dichromate. I have already obtained magnificent results and hope to do even better in the future.

Taking advantage of the method, found by me, of the black staining of the elements of the brain, staining obtained by the prolonged immersion of the pieces, previously hardened with potassium or ammonium bichromate, in a 0.50 or 1.0% solution of silver nitrate, I happened to discover some facts concerning the structure of the cerebral gray matter that I believe merit immediate communication.

What these two sciences of recognition, evolution and immunology, have in common is not found in nonbiological systems such as 'evolving' stars. Such physical systems can be explained in terms of energy transfer, dynamics, causes, and even 'information transfer'. But they do not exhibit repertoires of variants ready for interaction by selection to give a population response according to a hereditary principle. The application of a selective principle in a recognition system, by the way, does not necessarily mean that genes must be involved—it simply means that any state resulting after selection is highly correlated in structure with the one that gave rise to it and that the correlation continues to be propagated. Nor is it the case that selection cannot itself introduce variation. But a constancy or 'memory' of selected events is necessary. If changes occurred so fast that what was selected could not emerge in the population or was destroyed, a recognition system would not survive. Physics proper does not deal with recognition systems, which are by their nature biological and historical systems. But all the laws of physics nevertheless apply to recognition systems.