1922] Gardner: The Genus Fucus on the Pacific Coast 3 



to each kind fewer and fewer characters. Today, with the coming 

 of the geneticist, the concept of a species is becoming so limited that 

 for practical purposes, if carried to the extreme, it has but little value ; 

 nevertheless it cannot be ignored. It is now generally conceded that new 

 kinds of organisms have arisen, and are still arising through permanent 

 modifications appearing in the offspring which produce an individual 

 more or less different from the parent, and that these modifications 

 are heritable and stable. The causes of the modifications that bring 

 about such changes have not yet been assigned. According to Darwin, 

 the modifications are small and when the succession of infinitesimally 

 small modifications in the suppression or development of a structure 

 has proceeded long enough the offspring becomes a species. According 

 to De Vries. the modifications may be relatively great, so that one may 

 readily detect the difference between the parent and the offspring in a 

 single generation. The offspring becomes a new species at once. Pos- 

 sibly both these ideas are correct, but in some groups of individuals con- 

 stituting entities, the forces acting to bring about the changes operate 

 slowly, whereas in others they are accelerated and the resulting changes 

 are marked at once. In either case, it is left to the keenness of the 

 observer to detect the differences. And the difficulty does not end with 

 that ; it can not, in the nature of things, until a specific standard is set 

 up and agreed upon. Darwin never suggested by how much one group 

 should differ from another in order to constitute a species. 



Another complicating set of factors always enters into the situation 

 in a rapidly changing, or species making group, to further the difficulty 

 of detecting the limits of species. The individuals of every generation 

 of all groups of organisms may vary in one or more morphological 

 characters, but these differences are not heritable. This is spoken of 

 as fluctuating variation. The problem which one usually has to deter- 

 mine in placing an individual is whether the dominant character or 

 characters is due to fluctuating variation or to a change which has . 

 become fixed. The fluctuating variation may often be much greater 

 than the permanent difference between two species. Thus the average 

 width of the fronds of a species of Fucus (a) may be 10 cm., and 

 that of another (b) may be 15 cm. Some individuals in (a) may 

 possibly be wider than 15 cm. and some in (b) may be narrower than . 

 10 em. 



The causes of the differences in characters in fluctuating variation 

 may be grouped under two categories, one physiological and the other . 

 environmental. For example, two Fucus plants of the same lineage 

 may have equal environmental conditions and be equal in initial size, ; 



