100. General Observations
The principle of truly independent or convergent invention is more difficult to establish by positive examples than imitative diffusion. It has often been assumed as operative, more rarely proved; and even in the latter cases has perhaps never been found to lead to complete identity.
In fact, the first observation to be made is that resemblance must not be too close if independent development is to be the explanation. A complex device used in two or more parts of the world suggests a connection between them in very proportion to its complexity. A combination of two or even three elements might conceivably have been repeated independently. A combination of five or ten parts serving an identical purpose in an identical manner must necessarily appeal as impossible of having been hit upon more than once. One thinks almost under compulsion, in such a case, of historical connection, of a transference of the idea or machine from one people to the other.
If the resemblance includes any inessential or arbitrary parts, such as an ornament, a proportion that so far as utility is concerned might be considerably varied but is not, a randomly chosen number, or a name, the possibility of independent development is wholly ruled out. Such extrinsic features would not recur together once in a million times. Their association forces a presumption of common origin, even though it be difficult to account for the historical connection involved. The significance of names in this situation has already been commented on.
There is nothing arbitrary about this limitation on the parallelistic principle. We all apply similar checks in practical life. If in a court of law several witnesses testify to the same facts in the same language, without one of them adding or diminishing an item, if they follow the identical order of events, if even details such as the precise minute of an occurrence are stated without variation, judge and jury will infallibly suspect that the several testimonies go back to a single source of inspiration. Eyewitnesses will differ. They have seen from different angles; have followed events with attention that varied according to their participation and their previous habits and training; have reacted with individually colored emotion. So with nations. Their customs, interests, faculties are never wholly alike. Their independent inventions and innovations, always springing out of a distinctive soil, therefore necessarily take on a distinctive aspect even when they embody the same idea. In the degree that the form as well as the substance of culture traits coincide, does the probability of independent evolution diminish in favor of some sort of connection.