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. Zoologists 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 



