144 
PROGRESS OF MICROSCOPICAL SCIENCE. 
are dispersed black particles, sometimes angular, and a very few others 
of a rounded form and bronze colour. On examining the translucent 
grains with the polariscope, and under several different magnifying 
powers, it became evident they consist of felspar. The structure is 
reticulated and in a very few cases banded : but owing to the irregu- 
larity of the forms of the grains, I was unable to determine to which 
class of the felspars they are referable. My impression is that they 
are the dust of sanidine, and of a small proportion of plagioclase ; 
such, in fact, as would result from the pounding up of trachyte. The 
black grains are those of magnetite, and on placing a small magnet 
near the dust, a movement is immediately observed amongst the grains, 
which increases in intensity as the magnet approaches contact. 
The Law of Embryonic Development in Animals and Plants. — We 
take from the last number of the ' American Naturalist ' (July, 1875) 
an important letter, in which Mr. C. E. Dryer, of Ontario Co., N.Y., 
objects in very strong terms to the views expressed in a paper pub- 
lished by that journal (May, 1875). He says that the paper in^ques- 
tion opens with the startling proposition that " it is a well-known law 
in the animal kingdom, that the young or embryonic state of the higher 
orders of animals resemble (sic) the full-grown animals of the lower 
orders." If such a law had ever been discovered to exist, the tadpole 
and the caterpillar, which are cited in proof, would certainly be good 
illustrations of it. But this statement is so far from being " a well- 
known law," or " one of the causes of the recent rapid progress in the 
study of the animal kingdom," that no eminent living naturalist or 
biologist recognizes the existence of such a law ; or at least no one of 
them gives a hint of it in his writings. Agassiz claimed that ancient 
animals resembled the embryos of recent animals of the same class, 
and that the geological succession of extinct forms is parallel with the 
embryological development of existing forms. But if this principle 
be true, it is far from meeting the requirements of the " law " of this 
article. The writer of it may have had in his mind a vague idea of 
the law of Von Baer, which is well known, and which has enabled 
naturalists " to correct their systems of classification," viz. " That, in 
its earliest stages, every organism has the greatest number of charac- 
ters in common with all other organisms, in their earliest stages." Or, 
to put it in language parallel to that of the " law" of this article, false 
syntax excepted, the embryonic state of the higher orders of animals 
resembles the embryonic (not the full-grown) state of the lower orders. 
The germ of a human being differs in no visible respect from the germ 
of every animal and plant : it never resembles any full-grown animal 
or plant. It successively loses its resemblance to vegetable embryos, 
then to all embryos but those of Vertebrates, then to all but those of 
Mammals. Finally, it resembles only the embryos of its own order, 
Primates ; and at birth the infant is like the infants of all human 
races. But never at any period of its successive differentiations does 
it resemble the adult form of fish, reptile, bird, beast, or monkey. 
The principle stated is not a law of the animal kingdom. If it 
be a law at all, it is a newly discovered one, and applies only to the 
vegetable kingdom. 
