We cannot begin the study of the “causes” of the differentiation of form without a few words of explanation about the terminology which we shall apply. Causality is the most disputed of all categories; many modern scientists, particularly in physics, try to avoid the concept of cause altogether, and to replace it by mere functional dependence in the mathematical meaning of the term. They claim to express completely by an equation all that is discoverable about any sort of phenomena constantly connected.
I cannot convince myself that such a very restricted view is the right one: it is very cautious, no doubt, but it is incomplete, for we have the concept of the acting “cause” in our Ego and are forced to search for applications of it in Nature. On the other hand, it does not at all escape me that there are many difficulties, or rather ambiguities, in applying it.
We may call the “cause” of any event, the sum total of all the constellations of facts which must be completed in order that the event may occur; it is in this meaning, for instance, that the first principle of energetics applies the term in the words causa aequat effectum. But, by using the word only in this very general sense, we deprive ourselves of many conveniences in the further and more particular study of Nature. Would it be better to say that the “cause” of any event is the very last change which, after all the constellations necessary for its start are accomplished, must still take place in order that the event may actually occur? Let us see what would follow from such a use of the word causality. We here have an animal germ in a certain stage, say a larva of Echinus, which is just about to form the intestine; all the internal conditions are fulfilled, and there is also a certain temperature, a certain salinity, and so on, but there is no oxygen in the water: the intestine; of course, will not grow in such a state of things, but it soon will when oxygen is allowed to enter the dish. Is, therefore, oxygen the cause of the formation of the intestine of echinus? Nobody, I think, would care to say so. By such reasoning, indeed, the temperature, or sodium, might be called the “cause” of any special process of morphogenesis. It, therefore, seems to be of little use to give the name of cause to that factor of any necessary constellation of events which accidentally happens to be the last that is realised. But what is to be done then?
Might we not say that the cause of any morphogenetic process is that typical property, or quality, or change, on which its specific character depends, on which depends for example, the fact that now it is the intestine which appears, while at another time it is the lens of the eye? We might very well, but we already have our term for this sort of cause, which is nothing else than our prospective potency applied to that elementary organ from which the new process takes its origin. The prospective potency indeed is the truly immanent cause of every specification affecting single organogenetic processes. But we want something more than this.
We may find what we want by considering that each single elementary process or development not only has its specification, but also has its specific and typical place in the whole—its locality. Therefore we shall call the “cause” of a single morphogenetic process, that occurrence on which depends its localisation, whether its specific character also partly depends on this “cause” or not.[39]
This definition of “cause” in morphology may be artificial; in any case it is clear. And at the same time the concepts of the prospective potency and of the “means” of organogenesis now acquire a clear and definite meaning: potency is the real basis of the specific character of every act in morphogenesis, and “means,” including conditions, are the sum of all external and internal general circumstances which must be present in order that morphogenetic processes may go on, without being responsible for their specificity or localisation.
It is implied in these definitions of cause and potency, that the former almost always will be of that general type which usually is called a stimulus or “Auslösung,” to use the untranslatable German word. There is no quantitative correspondence between our “cause” and the morphogenetic effect.
Some Instances of Formative and Directive Stimuli
Again it is to Herbst that we owe not only a very thorough logical analysis of what he calls “formative and directive stimuli”[40] but also some important discoveries on this subject. We cannot do more here than barely mention some of the most characteristic facts.
Amongst plants it has long been known that the direction of light or of gravity may determine where roots or branches or other morphogenetic formations are to arise; in hydroids also we know that these factors of the medium may be at work[41] as morphogenetic causes, though most of the typical architecture of hydroid colonies certainly is due to internal causes, as is also much of the organisation in plants.