PROGRESSIVE STAGES IN COMPLEXITY OF STRUCTURE. 585 



Single cells of diatoms and desmids form similar social groups living within 

 restricted areas. We must suppose that here— exactly as in the young brood 

 produced from the spawn of a fish, which swim in company through the 

 water, or in midges hatched simultaneously, which dance up and down in the 

 evening sun— here must be some kind of family feeling which binds the different 

 individuals together, although we cannot fully comprehend these relations between 

 the several organisms. 



When the single genetically-connected masses of protoplasm, each retaining 

 its own individuality, can transfer themselves in common from one place to 

 another, like caterpillars, midges, grasshoppers, fishes and the like, the com- 

 munity is called a swarm ; if, on the other hand, the isolated individuals settle 

 quite close to one another on a substratum, and there take up a definite position 

 for their lifetime, then we speak of a colony. The amoeboid bodies of some 

 myxomycetes, several unicellular Palmellaceae, Desmidiese, and Diatomaceje, live 

 in swarms; the numerous Siphoneae, on the other hand, as well as the species 

 of the genera Synedra and Gomphonema belonging to the family of the Dia- 

 tomaceae, dwell in colonies (cf. vol. II., fig. 369 ^ and 369 "). These colonies often 

 attain to considerable dimensions. The Acetabularias attached to stones and 

 mussel-shells at the bottom of the sea, the swollen bladder-like Valonias, the 

 moss-like forms of Bryopsis, and the dusky species of Codium, form, arranged 

 in thousands side by side, very extensive colonies. The Vaucherias, dwelling on 

 damp earth and in cold springs, appear as extensive cushions which cover the 

 ground over a wide distance with a green felt. Besides the swarms and colonies, 

 we have a third form of assemblage, the cell-union, in which the genetically-con- 

 nected masses of protoplasm grow together /n a body. This union, again, differs 

 essentially according as to whether the individual masses of protoplasm forming 

 it are devoid of, or are surrounded by, a cell-wall. In the former case they are 

 fused into a mass in which the limits of the single individuals can no longer 

 be recognized, as is the case, for example, in many myxomycetes. The expres- 

 sion "fusion" can here be employed figuratively with the utmost propriety, for 

 indeed the process strongly resembles the fusing of fluid metallic globules 

 into a larger mass, or the fusing of numerous drops of oil floating on the 

 surface of water into a larger drop, in which the contours of the single fused 

 portions are obliterated. It is indeed doubtful whether the fused masses of 

 protoplasm do actually surrender their individuality. Certain phenomena tell 

 rather against than for this view. Thus many myxomycetes form so-called 

 sclerotia, i.e. they lose their mobility and pass into a temporary state of rest. 

 The whole mass becomes rigid, assumes a wax-like consistency, and dries up, 

 and the shapeless protoplasm divides into innumerable, clearly-defined, rounded or 

 angular particles. When at the end of the resting period the stiffened mass is 

 to be again transformed into the mobile condition, the individualized particles 

 become fluid and a fresh fusion takes place. The phenomenon observed in the 

 whole series of myxomycetes suggests the idea that the isolated corpuscles in 



