Page:Popular Science Monthly Volume 69.djvu/213

Rh thing that Dr. Merriam has put into print concerning his extensive and thorough field work in the west, and that I yield to none in my admiration of the wide inclusiveness of his results and the profitable manner in which he has treated incidentally the general features of the occurrence and distribution of plants. This appreciation is heightened by the fact that I have spent many seasons in the regions covered by him during the last fifteen years, and that I have carried on experimental work in the field and at the Desert Laboratory with many of the species which are included in his generalizations. I have not been able to come upon the evidence, or record of evidence, upon which his sweeping statements relative to plants are based, although detailed studies upon the relation of plants to environmental factors have been in progress for some time.

Dr. Merriam does not find any evidence to support the conclusion that species arise by mutation. It would be a matter of great surprise if he had. It would be as reasonable to have demanded of him the solutions of problems of respiration from his preparations and field notes. Once a mutant has appeared, no evidence of its distribution can be taken to account conclusively for its origin. Although I have had many mutants under experimental observation, I should not be able to speak with reasonable certainty as to the origin of any of them, had I not ascertained it by guarded pedigree-cultures. It also follows that the systematists who announce and describe new forms as mutants, simply from preserved specimens, or from individuals, the origin of which is not a matter of careful observation, are following a wholly unwarranted practise. Several months ago I had occasion to say "that the 'naturalists,' as some zoologists term themselves, having made the greatest number of essays to offer a universal interpretation of the problems of distribution, are to be credited with the greatest number of defenseless assumptions. In all genetic and evolutionary researches too much emphasis can not be laid upon the basal fact that the physiological and morphological natures of the two great classes of living things are so widely divergent that the derivation of universal biological principles from their apparently concurrent behavior must be made with the greatest caution." Nowhere does this find better exemplification than in the unedifying results of a recent discussion of isolation as a factor in evolutionary action. A number of zoologists have assumed to speak of the distribution of plants, with apparently no basis except 'general information' to the effect that closely related species do not have the same habitat. This has been variously put, but the general meaning is as given. Now such a conclusion is so widely inapplicable, and is so loosely guarded, as to be wholly without value as a statement of a principle in plant