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LECTURES ON EVOLUTION 103 



not represent the actual passage from the one 

 group to the other, as intercalary types, from those 

 linear types which, more or less approximately, indi- 

 cate the nature of the steps by which the transition 

 from one group to the other was effected. 



I conceive that such linear forms, constituting a 

 series of natural gradations between the reptile 

 and the bird, and enabling us to understand the 

 manner in which the reptilian has been metamor- 

 phosed into the bird type, are really to be found 

 among a group of ancient and extinct terrestrial 

 reptiles known as the Ornithoscelida. The re- 

 mains of these animals occur throughout the series 

 of mesozoic formations, from the Trias to the Chalk, 

 and there are indications of their existence even in 

 the later Palaeozoic strata. 



Most of these reptiles, at present known, are of 

 great size, some having attained a length of forty 

 feet or perhaps more. The majority resembled 

 lizards and crocodiles in their general form, and 

 many of them were, like crocodiles, protected by 

 an armour of heavy bony plates. But, in others, 

 the hind limbs elongate and the fore limbs shorten, 

 until their relative proportions approach those 

 which are observed in the short-winged, flightless, 

 ostrich tribe among birds. 



The skull is relatively light, and in some cases 

 the jaws, though bearing teeth, are beak -like at 

 their extremities and appear to have been enveloped 

 in a horny sheath. In the part of the vertebral 

 column which lies between the haunch bones and 



