Some evidence of a remarkably interesting kind has been collected by J. H. Powers[19] respecting the structure and habits of _Amblystoma tigrinum_, which led him to the conclusion that striking differences in the form, anatomy, and developmental processes could be effected directly by change in the conditions of life. It is well known that a profusion of forms, distinct in various degrees, is grouped round _Amblystoma tigrinum_. Some of these are believed to be geographically isolated, others occur together in the same waters, and, as usual, authorities have differed greatly as to the number of names to be given. These forms were studied in detail by Cope who described them in the _Batrachia of North America_. The view which he inclined to take was that the individual variations of _Amblystoma tigrinum_ resulted from variations in the time and completeness of the metamorphosis, and these were regarded as due to external causes, such as differences in season, temperature, and geographical conditions. Powers, however, states that collecting within a radius of six or eight miles he found almost if not quite the whole "gamut of recorded variation in this species." Some, however, as he states, occurred rarely except under experimental conditions, but considerable differences in temperature were not found necessary in producing them. Every year, he says, he has been able to add to the number of peculiar types found in the same small area in nature, until the amount of natural variation at least equals that seen by Cope in the collections of the National Museum and those of the Philadelphia Academy. Powers states that his observations by no means confirm Cope's view that these differences are in the main referable to variation in the completeness of metamorphosis, and on the contrary, he regards metamorphosis as on the whole a levelling process, tending to obliterate diversity. The enormous differences in size and proportions which he describes can only be appreciated by reference to his figures. They affect almost all features of bodily organisation. These striking differences he looks upon as brought about by differences in nutrition, "diversities in habitual locomotion," and diversity in the age at which metamorphosis occurs, and to sexual difference. Apart from sexual difference he regards the chief distinctions, in brief, as "acquired variations of the larva." As an example he gives the great elongation of some of the forms as "due first to slow growth, second to the free-swimming habit, third to the prolongation of larval life, and finally to the assumption of sexual maturity as males," either in the branchiate or non-branchiate condition. He describes the rapid growth of some and the slow growth of others. A larva of intermediate type may grow about a centimeter a month, but a rapidly growing specimen may grow more than four times as much. The slower rate of growth may, he says, be induced by winter feeding, and other treatment.[20]