272 



THE MICROSCOPE AND ITS REVELATIONS. 



number of cells in each, cluster, and the plan on which they are disposed, 

 are such as to baffle all attempts to base specific distinctions on such 

 grounds; and the more attentively the life-history of any one of these 

 plants is studied, the more evident does it appear that many reputed 

 'species' have no real existence. Some of these, indeed, are nothing 

 else than mere transitory forms; thus it can scarcely be doubted that the 

 specimen represented in Fig. 162, D, under the name of Pediastrum 

 pertusum, is in reality nothing else than a young frond of P. granu- 

 latum, in the stage represented in Fig. 161, E, but consisting of 32 cells. 

 On the other hand, in Fig. 162, E, we see an emptied frond of P. granu- 

 latum, exhibiting the peculiar surface-marking from which the name of 

 the species is derived, but composed of-no more than eight cells. And 

 instances every now and then occur in which the frond consists of only 

 four cells, each of them presenting the two-horned shape. So, again, in 

 Fig. 162, B and c, are shown two varieties of Pediastrum liradiatum, 

 whose frond is normally composed of sixteen cells; whilst at A is figured 

 a form which is designated as P. tetras, but which may be strongly sus- 



Various species (?) of Pediastrum. A., P. tetras ; B, c, P. biradiatum; D, P. pertusum ; s t 

 empty frond of P. granulatum. 



pected to be merely a four-celled variety of B and c. Many similar cases 

 might be cited; and the Author would strongly urge those Microscopists 

 who have the requisite time and opportunities to apply themselves to the 

 determination of the real species of these groups, by studying the entire 

 life-history of whatever forms may happen to lie within their reach, and 

 noting all the varieties which present themselves among the offsets from 

 any one stock. The characters of such varieties are diffused by the 

 process of binary subdivision amongst vast multitudes of so-called indi- 

 viduals. Thus it happens that, as Mr. Ealfs has remarked, "one pool 

 may abound with individuals of Staurastrum dejectum or Arthrodesmus 

 incus, having the mucro curved outwards; in a neighboring pool, every 

 specimen may have it curved inwards; and in another it may be straight. 

 The cause of the similarity in each pool no doubt is, that all its plants 

 are offsets from a few primary fronds." Hence the universality of any 

 particular character, in t all the specimens of one gathering, is by no 

 means sufficient to entitle these to take rank as a distinct species; since 



