No. 484] 



CLOSELY RELATED SPECIES 



225 



case of most flowering plants.^ The limitations seem to be those 

 of temperature, exposure, character of attachment, and to a less 

 degree, geological characters. Take ihv oumius \;nKlieria, for 

 instance. The last serious work is bv ( ioiz ; ;i >{\u\\ of tlie species 

 of Vaucheria in the neighborhood of Hast l. Switzerland. There 

 are 12 species there; 8 of them occur in Knijiand, G in the New 

 England States, 7 in California. Only three other fresh water 

 species are recorded for North America ; one is a P^uropean s})ecies, 

 found in the West Indies but not elsewliere so far on this continent; 

 the other two are from California. Now these two species, grow- 

 ing together, belong to the same subgenus, and I know of no 

 described species that I should say belonged in between them. 

 Take the genus Spirogv-ra. The best book on this is that of 

 Petit, Spirog}'res des Environs de Paris. He includes 37 species; 

 of these 34 have been found in North America. We have also 

 five other species; three of tliem are European, though not found 

 about Paris; the two others are from (ireenlaiid and Florida 

 respectively. It is mucli the same with all the fresli water alga-; 

 of the very inc<)iis|)icii()ns species, the records from (Hstant stations 

 are not so ahun(hint. bm tliat is laru'ely because the>e minute 

 forms have been little studied ont>ide of Knrope. 



greater. It would seem strange that marine alga- on the two 

 sides of the Atlantic, should dillVr nmch more than the fresh 

 water a1g;v of the two coiuinents. hut snch is the fact. Still the 

 resemblances are nmch greater than with flowering plants. 

 And there are many instances where clos(>ly allied species or 

 varieties have practically the same range. I will give a few such 

 pairs, and in each case there seeni> to be no species or variety 

 anywhere else that would stand between the two in question. 



"Cladostephus rnilcillafus and C. spon;/insus have the same 

 range, in temperate waters on both sides of the Atlamic. They 

 are the only species of the g(Mnis in that range. Fiicus (drntaftis 

 and F. evanescms have their hea.hiuarters in high arctic regions, 



' Compare Alph. Dc C:inaollr. ( in.i,naphie Botanique, 1, p. 499: "Nous 



d'autant plus petite que la classc dont elks font par k a une organisation plus 

 complete, plus dcveloppee, ou, selon Vexpression usU6e, plus parfaUe." 



