Hybridization between Fish Species in Nature

THE Jordan school of ichthyology, in which I was nurtured, held steadfastly to the view that the lines between fish species are almost never crossed. This mistaken idea was perhaps in part a holdover of the pre-Darwinian concept of the immutability of species, but was, more pointedly, a reaction against the tendency of some European ichthyologists to explain as hybrids specimens that proved difficult to identify. This view that there is little or no natural hybridization between fish species may be further attributed to the circumstance that systematic ichthyology in America from about 1890 to 1920 was still essentially in the exploratory and species-naming stage, and dealt chiefly with marine fishes, which, as I shall point out, hybridize much less often than do freshwater fishes. Early in my ichthyological career I encountered evidence that fish species do cross in nature, at times in considerable frequency. The analysis of such hybridization has ever since engaged my attention, in so far as other activities would allow. This report lays particular emphasis on the researches that I have conducted during the past thirty-five years, largely with the collaboration of my wife and of several students and colleagues, to all of whom I pay tribute. They might all appear as co-authors of this report. The studies continue. During the first ten years of my intensive studies of the freshwater fishes of North America, from 1919 to 1929, I gathered strong circumstantial indications that the species lines are rather often crossed in nature, and during the following fifteen years, from 1929 to 1944, I was able, with the constant aid of Mrs. Hubbs, to confirm these indications, not only by the accumulation of much more circumstantial evidence, in part subjected to critical statistical analysis, but also by breeding experiments in the aquarium of the Division of Fishes of the then new Museum of Zoology at the University of Michigan.