It may be that a given people tend to have a characteristic head-form, not so much because they are of common breed, as because they are subjected after birth, or at any rate, after conception, to one and the same environment.
Thus some careful observations made recently by Professor Boas on American immigrants from various parts of Europe seem to show that the new environment does in some unexplained way modify the head-form to a remarkable extent.

For example, amongst the East European Jews the head of the European-born is shorter and wider than that of the American-born, the difference being even more marked in the second generation of the American-born.

At the same time, other European nationalities exhibit changes of other kinds, all these changes, however, being in the direction of a convergence towards one and the same American type.

How are we to explain these facts, supposing them to be corroborated by more extensive studies?
It would seem that we must at any rate allow for a considerable plasticity in the head-form, whereby it is capable of undergoing decisive alteration under the influences of environment; not, of course, at any moment during life, but during those early days when the growth of the head is especially rapid.

The further question whether such an acquired character can be transmitted we need not raise again.