INTRODUCTION 19 



obtained from Europe, India, and North and South America 

 identical to the most minute microscopic detail ; and numerous 

 other equally stable forms might be cited. On the other 

 hand, in many instances intermediate forms occur whose 

 position is difficult to assign. In some cases it is possible to 

 regard climatic or seasonal conditions as the cause of 

 variation. The American and tropical examples of the genus 

 Cribraria are more elegant in form than individuals of the 

 same species found in Britain and on the Continent, and 

 most of them show a tendency towards the great regularity 

 typical of C. intricata, a striking and well-marked species 

 which is abundant in those regions, but rare in our less brilliant 

 atmosphere. The more elegant growth in the American species 

 is not confined to the genus Cribraria, but is of general occur- 

 rence ; and it is probable that this modification of type is 

 due to the influence of climate. This is what might be 

 looked for when we consider the effects which changes of 

 weather produce in the development of sporangia in this 

 country. On old decaying stumps which can be kept under 

 observation for several years, growths of Trichia affinis may 

 year after year present the same typical characters, only 

 differing in the elaters in one season being slightly thicker 

 than those in another. But should cold weather set in while 

 the plasmodium is rising, the arrangement of the spiral bands 

 may be so abnormal as to suggest a marked variety. With 

 a return of milder weather, however, the original form re- 

 appears, leaving no doubt that all have been derived from 

 a common parentage. In some extensive gatherings of the 

 same species which have matured in hot, dry weather, the 

 elaters are so reduced in size as scarcely to exceed the 

 diameter of a spore in length, though the sporangia are 

 perfectly normal in form and the spores are marked with 

 the typical sculpture. Developments of Trichia persimilis of 

 the typical form have been followed after a few nights' frost 

 by a growth in which the short and nearly smooth elaters 

 closely resemble those of Oligonema nitens, though the spores 

 and the shape of the sporangia retain the normal character. 

 In Stemonitis, Lamproderma, Prototrichia, and other genera, 

 great modifications are caused by changes of temperature. 

 But, after all aUowance has been made for such agencies, it 

 must be recognized that certain species are subject to 

 variation for which no cause can be given. 



In the systematic account of the Mycetozoa the descriptions 

 given are those of the most frequent type of each species. 

 Subordinate types representing distinct centres about which 

 examples group themselves, but connected with the main 

 type by gradations of character, are described as varieties. 



