1 INTRODUCTORY 13 



Nature's own experiments, cannot hope to arrive at sound in- 

 ductions, or to establish general laws of causation. 



There is, however, another way. Development, the production 

 of form, may be regarded as one of the activities, one of the 

 functions of the organism, to be investigated, like any other 

 function by the ordinary physiological method of experiment; 

 and the ideal of the experimental or physiological embryologist 

 is to give a complete causal account, whether the causes are 

 external or internal, of each stage, and so of the whole series 

 of ontogenetic changes, his weapon, to borrow Roux's splendid 

 phrase, ( die Geistesanatomie, das analytische causale Denken.' 1 



This effort is, of course, no modern one. Speculation into the 

 nature and essence of development begins, indeed, with the 

 Greeks, and theories of fertilization and development are to be 

 found in the writings of Aristotle. 2 In fertilization the male 

 element, which, according to Aristotle, provides the formal and 

 efficient causes in providing the necessary perceptive soul, acts 

 upon the mere matter, endowed only with a nutritive soul, which 

 is given by the female, in the same sort of way, to use his own 

 illustration, as rennet coagulates milk. In the germ thus formed 

 the parts of the embryo, which can only be said to pre-exist 

 potentially, arise not simultaneously but in gradual succession, 

 first the heart, then the blood, the veins from the heart and the 

 various organs about the veins by a process of condensation and 

 coagulation, the anterior parts of the body being built up first. 



This Aristotelian doctrine appears to have persisted through 

 the Middle Ages ; it reappears in the seventeenth century in the 

 pages of Hieronymus Fabricius ab Aquapendente and his pupil 

 William Harvey in essentially the same form, although both 

 authors differ from Aristotle in certain matters of observational 

 detail. Thus Fabricius 3 states that ' ope generatricis facultatis 

 pulli partes, quae prius non erant, produci atque ita ovum in 

 pulli corpus migrare', while Harvey 4 gives to development as 

 thus conceived of the name of ' Epigenesin sive partium super- 

 additionem', though he believes that in some cases (Insects) the 



1 Roux, 1885. 



2 Aristotle, De Gen., i. 20. 18; ii. 4. 43 ; 5. 2, 3, 10 ; 6. DeAn., ii.4. 2, 

 15 ; 5 ; 6. 



3 Fabricius, 1. c., p. 22. 4 Harvey, 1. c., Ex. 44. 



