INTRODUCTORY 9 



bator, is incredible, we say the ^g% is developed by its inherent 

 potency ; but we must use these words with care, for the assertion 

 that the changes which make up this long and marvellous series 

 take place spontaneously is as incredible as the assertion that they 

 are determined by the heat of the hen ; and there is reason to believe 

 that each change in the series transmits to the natural or inherent 

 adaptive mechanism a stimulus which excites in it the performance 

 of the responsive actions which bring about the next change in 

 order. 



Embryonic development is so delicate and so complicated that 

 we cannot hope to trace, far less to imitate, the action of these 

 stimuli in anything like their natural perfection ; yet we can, now 

 and then, rudely imitate some of them, while, in other cases, we can 

 demonstrate their presence and influence indirectly by preventing 

 them from acting. Some eggs which have begun their development 

 by division into two, four, or eight cells, may be shaken apart with- 

 out destroying their vitality, and when thus separated, a cell which 

 would normally have given rise to half or quarter of an embryo, may 

 give rise to a whole one of one half or one quarter the natural size. 

 Embryologists are rapidly adding, by experimental methods, to our 

 knowledge of the mechanics of development, and it has been known, 

 since the day of Aristotle, that some of the latest stages in the 

 development of the higher animals and of man do not take place in 

 the absence of certain normal physiological stimuli. 



Male mammals, for example, do not attain bodily perfection until 

 the approach of sexual maturity. In man the beard begins to grow 

 at what is accordingly called the age of pubescence ; the larynx 

 enlarges ; the voice assumes a manly tone ; the shoulders grow 

 broad ; the chest deepens ; and the trunk and limbs begin to differ 

 in relative length from those of women and children. At the same 

 period in the life of a bull his neck and shoulders grow massive and 

 sturdy ; his forehead broadens and becomes cushioned with hair ; and 

 he becomes pugnacious and subject to fits of violent rage. 



The cock acquires his spurs, his brilliant plumage and other 

 ornaments, and begins to crow. Aristotle pointed out that when 

 young male mammals or birds are prevented from becoming sexually 

 mature, they fail to acquire the distinctive characteristics of their 

 species, and this shows that the completion of this, the final stage 



