The isolation of sea-going Species in small lakes, as appears to have taken place in these instances, poses a problem in species delimitation. The alternatives lie between describing each isolate as a distinct species or subspecies, or including all the populations in a single, widely variable species, derived from the parent sea going species. Since there has been a fundamental change in the life history pattern of the land-locked species, it seems likely that the parent and derivative Species have acquired reproductive iso lating mechanisms. This conclusion is supported by the fact that the derivative populations have developed characteristic morpho logical differences from the parent population. The specific dis tinctness of the two derivative populations under examination is another problem. Ideally, breeding experiments and behavioral studies of the two populations would be desirable to provide the answers, but these are outside the sc0pe of the present study. On examining the morphological differences between the two popula tions, it seemed likely that they were independently derived from the parent stock and therefore more closely related to the parent than to each other. The population in Lake Rototuna is, from both morphological and geological considerations, probably an earlier derivative of G. Maculatus attenuatus than is the Lake Waiparera population. Lake Rototuna occurs in well stabilized rolling sand dunes, once covered in bush, at an altitude of 300 ft. (91 m), whilst Lake Waiparera is in still shifting sand dunes, about a hun dred feet (36 m) above sea level. Corresponding to their apparent difference in age, and perhaps to differing ecological conditions in the two lakes, the degree to which these species have diverged from their common parent species differs greatly. The differences are most obvious in vertebral count, a character which has proved useful in separating Galaxias species in New Zealand. Assuming relationship to G. Maculatus attenuatus, the Lake Rototuna popu lation has traversed the full range of vertebral number in the New Zealand Galaxiidae, from the maximum in G. Maculatus attenuatus of 59-64, to a minimum of 47-50. The vertebral number in the Lake Waiparera population is also reduced and almost completely disjunct from both G. Maculatus attenuatus and the Lake Rototuna population, viz. 54-59 (fig.

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