May 1, 1899.] 



KNOWLEDGE. 



115 



presence of colours other than green is found a funda- 

 mental character in their classification, and in like manner 

 we shall find in the mysies that the colour of the spores 

 has been found a character of real value. 



What is the meaning, some one may ask, of the value 

 of a character for classLficatory purposes ? It means that 

 the presence of that character affords a safe line of cleavage ; 

 that those plants or animals which are on one side of the 

 line will be found to agree in other characters — will have 

 a likeness in many points of that kind which creates what 

 we call in human beings a family likeness ; whilst those 

 organisms which stand on the other side of the line will 

 be found dissimilar from the first family group. For 

 instance, if we gather the common white dead nettle 

 and observe its stalk, we shall find that it is four-sided, so 

 that a section across it is a square. Now this characteristic 

 might easily be supposed to be one of little consequence, 

 and yet, in fact, it will be found to be a true and valuable 

 one, and that all plants with a square stalk and lipped 

 flowers will be found to have a four-lobed ovary and four 

 nuts on the bottom of the calyx, and these belong to the 

 family of the LabiatiP. If now, on the other hand, we 

 count the number of the stamens in plants, and use this 

 character as the foundation of our classes, we shall break 

 up this natural family with its square stems, and shall 

 relegate some genera, such as Salvia, to one class, while the 

 great mass of the family go to another, and, what is 

 perhaps worse, these exiled genera find themselves put 

 into a class together with plants with which they have no 

 real connection or sympathy — with the Enchanter's 

 Nightshade and the Duck-weed. This form of the stem 

 then has a high value as co-existent with a general likeness 

 of structure ; the number of the stamens may vary in 

 plants closely akin, and agree in plants widely different, 

 and therefore has a low systematic value. 



The variations of form of our domesticated dogs are 

 generally held to be of no value even as specific distinctions ; 

 but the difference of the markings in the spores of myxies 

 is held by those who have most studied their classification 

 to be often a safe difference as between two species. It is 

 only by experience that we can tell the systematic value 





Fro. 5. — Cribraria auranUaca x about sixty diameters. 



of a difference — i.e., by observing how far it is correlated 

 with other differences of structure or life-history, and 

 whether the difference does, or does not, lose itself in a 

 series of easy gradations between the two extreme forms. 

 And yet there are some minds whose thoughts so run 

 along the lines of creative thought that, as if by a happy 



intuition, they are able to seize these crucial points which 

 are of real value, and to reject those that are useless. 

 Such is the mind of the true naturalist. 



Some slight difference exists amongst naturalists as to 

 the extent to which the group of the Myxomycetes is to be 

 carried — viz., whether they shall include or exclude a 

 small group of organisms about to be mentioned, and as to 

 the way in which the two terms Mycetozoa and Myxomycetes 

 shall he used in classification. The following table may 

 be useful as indicating the primary and secondary divisions 

 of the group, which we shall accept in its widest significa- 

 tion : — 



Example, 

 J Dicfi/ostelium. 



f With an aggregate plasraoilium, 

 Acrasiem 



Jlveetozoa 



fExosporese, 



With 



a fused plasmo- j 

 dium. Mi/xomycetes 



borne' er- \Ceratomyxa. 

 ternally. ] 



Arctfria. 



Endosporete 

 spores 

 borne in- 

 L I, ternally. J 



Of these classes, it may at once be observed that the 

 endosporous Myxomycetes are by far the largest, and that 

 the species at present known of the other groups are very 

 few in number, and, accordingly, in the sketch which we 

 have given of the life-history of a myxie we have dealt only 

 with the changes in an endosporous myxie. 



It now becomes needful to call attention to the points in 

 which the smaller classes dift'er from the dominant one. 



In the ordinary myxie, as we have seen, the swarm 

 spores effect a true fusion and build up one mass of proto- 

 plasm. In the Acrasieii', on the contrary, the swarm 

 spores do not fuse or coalesce together, but only aggregate 

 together, retaining a power of separating from and moving 

 on one another. This is the first and broadest division of the 

 group of organisms. 



The next character- 



istic which has been ^, ^ 



used for the classifica- 

 tion of the group is the 

 position of the spores 

 in the organism. 

 Hitherto we have only 

 mentioned spores as 

 contained within the 

 sporangium ; but 

 there are one or 

 perhaps two species 

 very different in many 

 ways from the rest of 

 the group, in which 

 the spores are carried 

 on the outside of the 

 organism. From this 

 character the whole 

 myxomycetes have been divided into two classes : the 

 Exosporea', in which the spores are developed on the outside 

 of the sporoplmre — i.e., the part of the organism which bears 

 the spores ; and the Endosporeae, in which the spores are 

 generated within the sporangium. 



'm. 



7-- , 



■:^ 



0m 



Fid. 6. — Arci/ria punicea (cap = capil- 

 litium ; c — cup ; p = pedicel) x about 

 ten diameters. 



(a) Note. — To avoid confusion, it may be well to state that in the fore- 

 going table we have followed the classification of De Bary — that Van 

 Tieghem woidd write " Myxomycetes " as the name of the whole class 

 where we have written "Mycetozoa," and woiild write "Myxomycetes 

 proprement dits " where we simply write " Myxomycetes " ; and that 

 Mr. Lister uses "Mycetozoa " for what we have called "Myxomycetes," 

 and 30 excludes the Acrasieie from the Mycetozoa. 



