326 NATURAL SCIENCE. Nov., 



twenty bodies called dictyosomes, just as the fusion of the bubbles in 

 the nucleus swept together the nuclear chromatics into the chromo- 

 somes. Thus pseudosomes, centrosomes, and dictyosomes, as well as 

 chromosomes themselves, are all expressions of the froth-like activity 

 of protoplasm. 



It is of much interest to find an observer of actual miscroscopic 

 details in karyokinesis connecting the actual observed processes 

 with, on the one hand, Professor Biitschli's Foam Theory of proto- 

 plasm, and, on the other, Professor Weismann's idants and reducing 

 divisions. It is truly bringing the phenomena of heredity into close 

 relation with physical phenomena. 



The Origin of Dicotyledons. 

 Botanists as well as geologists will welcome Professor 

 Lesquereux's posthumous monograph on the Flora of the Dakota 

 Group (Monographs of the United States Geological Survey, vol. xvii.). 

 Few facts are so puzzling to the evolutionist as the sudden appearance 

 in profusion in Middle Cretaceous times of highly-organised 

 dicotyledons, often belonging to existing genera. The older strata, 

 even the Wealden, give no hint of the existence of any plants of such 

 high type ; and yet the Cenomanian deposits of Dakota have already 

 yielded no fewer than 429 species of dicotyledons, without including 

 any herbaceous species, for little has been preserved except leaves of 

 deciduous trees. When the discovery in Dakota of these remarkable 

 deposits was first announced, there was a natural hesitation to accept 

 their Cretaceous age ; but, since the stratigraphical evidence has 

 been made clear, botanists have been awaiting the appearance of 

 figures and descriptions, to show to what extent the reference of so 

 many of the plants to still living genera is justifiable. Now that this 

 handsome monograph has been issued, we are better able to judge, 

 and one cannot help being struck with the close resemblance of many 

 of the leaves to those of existing plants ; yet, on the other hand, one 

 observes that nearly all of the species are described from leaves alone, 

 and that the few detached fruit yet found have nearly all to be left in 

 the indefinite genus Carpites, for they do not seem to be closely allied 

 to recent genera. According to Professor Lesquereux's analysis, the 

 flora of the Dakota group is composed of 460 species, of which 6 are 

 ferns, 12 cycads, 15 conifers, 8 monocotyledons, and 429 dicotyledons. 

 Such a proportion of dicotyledons is far more suggestive of a recent 

 flora than of one of Cretaceous age, and it is singular how few charac- 

 teristic Secondary genera have been obtained. Professor Lesquereux's 

 suggestion, that we are enabled to refer the origin of the dicotyle- 

 donous plants to the beginning of the Cretaceous period, no doubt 

 accords with the facts, as far as they are yet known ; but both 

 botanists and geologists will hesitate to adopt it, in face of the highly 

 differentiated dicotyledons existing in the Dakota group. 



