78 THE AMERICAN NATURALIST. [VOL XXXIII. 
the investigator must be in drawing conclusions from mere impres- 
sions, no matter how clear they may be. The extraordinary external 
resemblances between unrelated forms is shown by a number of illus- 
trations; e.g., Equisetum, Casuarina, Ephedra. 
The second part of the volume is systematic and deals with the 
fossil forms from the lowest plants, Bacteria and Schizophycez, and 
other simple unicellular organisms, and comprises the Algæ, Fungi, 
Mosses, and part of the Pteridophytes. 
Among the lowest forms, as might be expected, fossils are 
comparatively few, and often of more than doubtful authenticity. 
Nevertheless, there are evidences of the existence of Bacteria and 
blue-green Alga in very old formations, although in most cases these 
evidences consist rather in the results of their growth than in the 
presence of the organisms themselves, e.g., calcareous nodules formed 
by Cyanophycez, partially decomposed tissues attacked by Bacteria. 
Among the Algze, only such forms as have a calcareous or silicious 
skeleton have left fossils whose nature is unmistakable. Professor 
Seward is very cautious about accepting the validity of many such 
doubtful forms as the so-called “ Fucoids,” for instance, which are 
merely impressions of extremely doubtful character. There are, 
however, very perfect fossils whose algal nature is perfectly evident. 
Of these, the Diatoms, the calcareous Coralline Algz or “ Nulli- 
pores,” and the calcareous green Algæ, belonging to the Characee 
and Siphonez, are the best known. The latter group and the Cor- 
allines have played an important part in the formation of certain 
limestones. As at the present day, in the warmer seas, they grew 
about the coral reefs which owe their growth to no inconsiderable 
degree to the activity of these calcareous Algae. According to Pro- | 
fessor Seward, there are reliable evidences of the existence of Cor- 
allines as far back as the upper Cambrian and lower Silurian, and 
the calcareous Siphonew were evidently abundant in the ancient 
seas. The Diatoms are a much more recent development, and no 
certain evidences of their existence have been found below the 
secondary formations. 
The true Phzophycex, or brown Algae, never develop a calcareous 
or silicious skeleton, and have left practically no fossil remains that 
are certainly recognizable. “Many casts from the older formations 
have been attributed to Algz of this group, but these casts are usually 
of very doubtful origin. The most probable remains of Pheophycez 
belong to the gemus Nematophycus, from Devonian and Silurian 
deposits. This genus shows some evidences of a structure compara- 
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