THE EVOLUTIONARY HISTORY OF PLANTS 299 
common ; the impressions of fern-leaves in bituminous coal 
and pieces of wood turned into a flint-like substance are 
two of the best known examples. 
The only way in which we can get knowledge about 
the animals and plants that inhabited the earth’s surface 
before men did is by studying such rocks as contain the 
remains of living things. In this way a great deal of 
information has been gained about early forms of animal 
life and a less amount about early plant life, — less because 
as a general thing plants have no parts that would be 
as likely to be preserved in the rocks as are the bones 
and teeth of the higher animals and the shells of many 
lower ones. 
370. The Law of Biogenesis. — An extremely important 
principle established by the study of the development of 
animals and plants from the egg or the seed, respectively, 
to maturity is this: The development of every individual is 
a brief repetition of the development of its tribe. The prin- 
ciple just stated is known as the law of biogenesis. As 
eggs develop during the process of incubation, the young 
animals within for a considerable time remain much alike, 
and it is only at a comparatively late stage that the wing 
of the bird shows any decided difference from the fore-leg 
of the alligator or the turtle. Zodlogists in general are 
agreed that this likeness in the early stages of the life 
history of such different animals proves beyond reasonable 
doubt that they all have a common origin, that is, are 
descended from the same kind of ancestral animal. 
Among plants the liverworts and ferns supply.an excel- 
lent illustration of the same principle. In both of the groups 
the fertilized egg-cells, as the student may have learned 
