FORMATION OF THE HUMAN HAND. 167 



is here said respecting Eespiration is very questionable, 

 but the general idea of an increasing specialisation of 

 function, in the increasing separation of the parts, is well 

 expressed. It is indeed a fundamental law that every 

 advance in complexity of organisation takes place 

 through a gradual differentiation, or specialisation, of 

 the general envelope. These important synonyms, 

 differentiation and specialisation, I will explain by 

 illustrating the law to which they point — namely, the 

 law of animal development first enunciated by 

 Goethe, and strikingly applied by Von Baer * : — 

 Development is ahvays from the General to the Par- 

 ticular^ from the Homogeneous to the Heterogeneous^ 

 from the Simple to the Complex; and this by a 

 gradual series of differentiations. 



When we say an organ has been formed out of a 

 tissue, we say a differentiation has taken place ; and 

 the function, e. g. resj)iration, which before was per- 

 formed by the general tissue, is now specialised, i. e. 

 performed by that special organ. A homogeneous 

 mass of organic matter, such as the Amoeba, which has 

 no organ whatever, performs all the functions of Assi- 

 milation, Respiration, Locomotion, and Reproduction, 

 by its general mass, not by any special organs. 



The process of differentiation by which special or- 

 gans are gradually developed in the ascending scale 

 of the animal series, is equally exhibited in any parti- 

 cular case of development. Thus if we follow the 



* Goethe : Werke, xxxvi., Zur Morplwlogie, 1807. Von Baer : Zut 

 Enhoiclcelungsgeschichte. 1828. I. 153. 



