38 
POPULAR SCIENCE REVIEW. 
life of the globe is interrupted by enormous periods of time. 
This is due to the fact that the greatest number of the rocks 
with which geologists are best acquainted were formed in seas 
far from land, where it was impossible for land vegetation to 
be carried without destroying its distinguishing characters. 
Notwithstanding this 44 imperfection,” however, there are 
certain broad and unquestionable principles which geologists 
can depend upon in the succession both of animal and vegetable 
types. Thus we find that the simplest forms appear in the 
oldest formations, and the most complex and highly organized in 
the latest. No investigation appears likely to disprove this 
most important fact. Again, as regards animals particularly, 
and, far as can be ascertained, with plants also, the most 
generalized types of organized objects precede the specialized 
forms. If we had as good means of identifying plants from 
fragments as we have in the case of the higher animals, there 
can be little doubt we should find the same truth holding equally 
good in the vegetable kingdom. Modern naturalists have made 
some important discoveries by the light of embryology — that 
is, of the probable descent of any species by tracing the successive 
changes which it may pass through as an embryo. In this way 
it is found that the embryo of a mammal passes through an 
enormous number of stages, from those representing single-celled 
infusorial animalcules upwards and onwards through others 
corresponding to fish, amphibian, reptile, and bird. These 
embryonic stages are a kind of condensed evolution, taking place 
and succeeding each other in a few months instead of in millions 
of years, and occurring in the life history of one animal instead 
of an incalculable series. Embryology therefore is regarded 
as illustrative of palaeontological evolution. 
But here again animal structures give a greater advantage 
to the student of their history and relationships than plants. 
The latter have fewer embryological stages through which we can 
trace their probable lines of descent. This has been attempted 
by a few English and German naturalists, and with considerable 
success, but the subject remains an almost un worked field. The 
most suggestive probabilities as to the origin of some of the 
leading orders of flowering plant come from a quite unexpected 
quarter, namely, their monstrosities or teratology. Thus,- we 
find that frequently the abnormal conditions or monstrosities 
of one order of plants correspond to and assume many of the 
characters distinctive of another order. In this way it is 
possible some orders may have arisen through adventitious 
changes in structure brought about by changes in the physical 
surroundings, such as climate, moisture, and height above the 
sea-level. 
The correspondence in the mode of succession of animal and 
