vironment, which selected those mutations capable of surviving.
Without question, de Vries had a real basis for his theory. Mutations do take place among both wild and domestic creatures; thus among the dandelions there constantly appear special types which breed true and are, as Castle has called them, "little species within the dandelion species." Similar mutations are well known in peas, beans, evening primroses, and such domestic animals as the sheep. Clearly, therefore, species do arise as de Vries stated; the question is, is this the only way in which they arise?
This problem was raised little more than twenty years ago—a period far too short to allow for the settling of a question that is merely another statement of the problem that has puzzled scientists and philosophers for more than twenty centuries.
There is, however, excellent reason for believing that the conceptions of both de Vries and Darwin are true; that neither of them excludes the other from operation. Thus in the famous chalk formation of England there may be found an evolutionary chain of sea-urchina which, according to the general concensus of opinion, represent true Darwinian evolution. As N. C. Macnamara says, "They are first found in their shelled, sparsely ornamented forms, from which spring, as we ascend the zone, all the other species of the genus. The progression is unbroken and minute in the last degree. We can connect together into continuous series each minute variation and
