The Easter Symposium of the British Ecological Society on 'The Ecology of closely allied species' (Anon. I944) considered Gause's contention (I934) that two species with similar ecology cannot live together in the same place. Elton published his contribution to this meeting in an expanded form (Elton, I946), giving an analysis of 55 animal and 27 plant communities, and calculating the number of species per genus for each community, the average of which was found to be i 38 for animal, and I'22 for plant communities. Elton also considered the number of species per genus in faunal lists, taking A check list of British insects (Kloet & Hinks, I945) as an example. In this case he calculated the number of species per genus to be 4-23. He attributed (p. 66) 'the differnce in species/genus frequencies between ecological surveys of relatively small parts of any general habitat, and those for faunal lists from larger regions,. .. to existing or historical effects of competition between species of the same genus, resulting in a strong tendency for the species of any genus to be distributed as ecotypes in different habitats, or if not, to be unable to co-exist permanently on the same area of the same habitat.' This paper was criticized by Williams (I 947) who, using a statistical method based on the logarithmic series, re-analysed 13 of Elton's communities and showed that they contain fewer genera than would be expected in a sample of the same number of species taken at random (i.e. independent of generic relations) from a suitable fauna or flora list. This, he concluded (p. I7), 'can only be interpreted as a natural selection-in the course of time-in favour of species in the same genus rather than against them. It is possible to suggest reasons for this-for example, if one species in a genus is capable of survival in a given physical environment, it seems likely that other species in the same genus might be more likely to have a similar genetic make-up than species in another genus, and so might have a good chance of survival' Before attempting some closely related work, it was found necessary to attempt a reconciliation between these two conflicting views, and the present note is the result of a consideration of the thirteen communities used by both Elton and Williams from the point of view of their suitability for the type of analysis to which they have been subjected; from this consideration three important points have emerged. First, the habitats of many of the communities are very heterogeneous, in some cases including species from several totally dissimilar minor habitats. For example, the list analysed as community 20 (community numbers refer to Table I, p. 55 of Elton, I946) was compiled from samples taken from five stations chosen over a 5o-mile stretch of the River Wharfe (Percival & Whitehead, I930). The distances between adjacent stations varied from 6 to 26 miles; and from the first station at Grassington to the last at Ullskelf the river changes from a fastflowing mountain river with a rocky bottom, to a placid alluvial lowland river. The list on which community i 2 was based was derived from a survey by Pyefinch (I937) of eight pools on Bardsey Island, ranging from brackish to fresh-water conditions. Pyefinch stresses (p. I37) 'the great variation in the environment from one pond to another, as each is influenced to a considerable extent by purely local conditions'. Indeed he states (p. I I7) that the ponds were 'so chosen that the full range of environment was adequately represented'. Similar criticisms apply to community I3 (Laurie, I942) and also to community 14 (Humphries, 1936) where, in the original survey, particular attention was paid to two stations as they represented physiographically different habitats. In these cases Elton and Williams have considered the occupants of a variety of habitats as constituting one community, even when the authors of the original papers have gone out of their way to stress that the sampling stations were chosen expressly because of their dissimilarity. Both authors were interested in animals living together in one habitat in competition with each other, so that the choice of many of the communities, especially the Bardsey Ponds with their great physical and chemical differences making them quite unsuitable for the type of analysis to which they were subjected, was particularly unfortunate. Secondly, it should be noted that the animals in the original lists were often collected by means of a variety of sampling devices. For example, the