300 THE EVOLUTIONARY CAREER 



Equus — the modern horse with i large toe and 



vestiges of two 

 others ... A 



///// 



and the time order is as follows : 



A >A' ^ A' ' >A.' ' ' >K" ' ' > A' " " 



Lower Eocene Early Late PHocene Pleistocene 



Eocene Miocene Miocene 



--^Time about 60 millions of years > 



This is a major paleontological sequence : the changes in morpho- 

 logy between stage A"" and stage A'"", for instance, are generic 

 ones, corresponding roughly to the difference between a dog and 

 a wolf. 



Minor paleontological sequences occur and the magnitudes of 

 the changes between stage and stage in them are of about the 

 same value as those small changes called Mendelian ones. 

 Examples are the changes in form of the septa in the Mesozoic 

 Ammonites, or in the pores in the tests of sea-urchins of the 

 same systems. In these latter sequences we probably see the 

 actual steps of the evolutionary process — the elements of trans- 

 formism themselves in permanent record. In the major sequence 

 illustrated above we see, between stages A"" and A'"", that is, 

 between Hipparion and Equus a morphological change in which 

 the 2nd and 4th toes became reduced to the vestigial bones that 

 we find in the present-day horse. We do not suppose that this 

 change occurred all at once, that is, that a three-toed horse had 

 offspring showing only one functional toe. Either on the hypo- 

 theses of Lamarckism, or on those of natural selection of small 

 inheritable variations there must have been a long series of genera- 

 tions between Hipparion and Equus and in these generations the 

 anatomy of the feet varied by very small steps — just such small 

 steps as are seen in the series of Ammonities, or of sea-urchins 

 mentioned above. In the course of this minor sequence the 

 2nd and 4th toes gradually diminished in size until they became 

 small vestigial bones and at the same time the 3rd toe became 

 gradually bigger until it became the functional walking one. 



99Z>. Phylogenetic Histories. Such a series of forms, 

 beginning with the Eocene Phenacodus and ending with the 

 existing Equus, represents the phylogeny of the Horse, an Ungulate 

 mammal which has specialized in a certain way (increasing size, 

 tending to run speedily on great, grassy plains, exchanging 



