Botany and Zoology. 419 
In this connection it should not be forgotten that the power of the 
best microscopes is limited, and that objects of less than a determinable 
size, are still invisible. Lines ruled on glass by mechanical means, cease 
to be resolvable by the best lenses when the distance between them ap- 
proaches ‘00001 of an inch, and there is no good reason to suppose that 
living germs may not exist far more minute than this, 
We have dwelt upon these sources of doubt with regard to such ex- 
periments because the leading arguments of the first part of Prof. Clark’s 
work are based almost wholly upon the results of Prof. Wyman’s exper- ° 
iments. We would not be understood as asserting that the spontaneous 
origin of living beings under such circumstances is impossible ; but only 
that we need additional evidence. 
Among the most interesting portions of the work are the explanations 
of the origin and early condition of ovarian eggs, and the changes they 
undergo pment proceeds. The egg considered as the lowest 
de 
the lowest animals,—and 
development of the egg or embryo is shown to depe t 
secondary ca most so amon mals, in this re- 
§ comparison is made with those germ-like forms supposed to 
Dn 
a comp ris 
originate and develop wholly through secondary causes. 
