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LECTURES ON BIOLOGY 



difference being their more delicate and graceful structure, their 

 smaller size, which is much less than that of the zebra, and finally 

 the vestiges of the lateral digits which do not now touch the 

 ground. Living together in large herds, these animals were widely 

 distributed, remains having been found in the Tertiary strata of 

 Europe, Asia, Africa, and even the New World (North America). 

 The more we approach the present day, the more rudimentary 

 the toes become, finally to disappear completely in the modern 

 horse (fig. 29). But not only the legs of the Equidae have been 

 modified ; the teeth, too, have undergone a great change, the 

 original short root teeth having developed into long prismatic 

 teeth (fig. 30). 



3 -^ The palseontological proof, though 



sufficiently strong in itself to convince the 

 profoundest sceptic, is corroborated by 

 other evidence. Occasionally animals are 

 born with one or two supernumerary toes 

 on the anterior or posterior feet. Such 

 reversions to a previous phylogenetic 

 stage are described as atavism. We 

 shall hear of similar instances later on. 



While we found in the Jurassic 

 period the first bird-like animals, we find 

 in the Cretaceous strata typical birds, 

 as, for instance, the Hesperornis, a bird 

 of the size of a stork and the shape of 

 a penguin. Mammals, too, begin to play 

 an important part and to resemble in 

 their development the present forms. 



If we now draw our conclusions, it is above all remarkable 

 that the very first animals observed in the oldest stratum in 

 which life existed, the Cambrian Period, are comparatively highly 

 organized Trilobites and Brachiopods. But, on the other hand, 

 we perceive a gradual continuous development from the simple 

 to highly complex forms which is quite incontestable. We are, 

 therefore, forced to assume that the Cambrian is not the first 

 period during which life existed, but that low forms of life must 

 have existed in preceding periods. Indeed, some of the most 

 eminent palaeontologists say that the preceding period of more 



FIG 30. TOOTH-DEVELOP- 

 MENT OF THE HORSE. 



(1) Tooth of Anchitherium ; 



(2) of Hipparion ; (3) of 

 Equus (half natural size). 



(After Kayser.) 



