EVOLUTION AND GENERATION 257 



portion vvliich passes through the umbilical arteries reaches the placenta 

 and becomes oxidized and otherwise improved by the interchanges which 

 take place between the maternal and fcetal Huids. 



Development of the Organs of Special Sense.— As soon as 



the structures forming the bony boundaries of the spinal column and 

 cranium are ready for their reception, the spinal cord and the brain are 

 formed, constituting the cerebro-sj)inal system r the eyes, organs of 

 hearing and taste, are gradually developed by the ordinary processes of 

 cell-formation. 



The alimentary canal has already been referred to in connection with 

 the mucous layer of the blastodermic membrane, and it may ])e observed 

 that from the same source the other organs of the abdominal cavity, 

 and also the organs of respiration, are formed, and the foetal structures 

 are, so far as general outline is concerned, complete. The subsequent 

 pi'ocesses are those of growth due to the continually added supplies of 

 nutriment, until the young animal is fit for " separate life ", when some 

 mysterious stimulus acts upon the uterus and causes expulsion of the 

 foetus in the act of parturition. 



The Foal. — For some time, however, the foal has to depend on its 

 mother for its subsistence, and as soon as it can rise to the erect 

 position, instinctively it seeks for the teats of the dam, from which for 

 some months to come it will obtain its chief food. 



By degrees the foal, prompted by instinct or curiosity, essays the 

 taste of the herbage at its feet, and in time begins to prefer it to the 

 maternal fluid. The mother at the same time seems to realize tliat her 

 nursing days have been sufKciently prolonged, and gives her colt em- 

 phatic hints that it has ceased to be solely dependent on her for its 

 daily food. 



EVOLUTION AND GENERATION 



The story which has just been told of the function which is ex- 

 pressed in the term " generation " may be considered from more than one 

 point of view. 



In the record of the successive changes which end in the production 

 of the young mammal resembling its parents in form, constitution, 

 temperament, and susceptibilities, the scientist sees an example of the 

 process of evolution, so familiar that it fails to excite any special notice. 

 The less experienced observer, with a larger perception of the marvellous, 

 cannot avoid being impressed with the remarkable results of apparently 

 simple causes. Opponents of the theory of evolution have found some 



