xi THE ELEMENTS OF STRUCTURE 205 



in which we use the word in reference to the parts of 

 multicellular animals. The term organelle may be kept 

 for the specialised parts of a Protozoon or of a cell, 

 leaving the word organ for the specialised parts, com- 

 posed of many cells, in Metazoa. (See Fig. 51.) 



Organs are well-defined parts, such as limb or liver, 

 heart or brain, in which there is a predominance of one 

 or a few kinds of vital activity. It is gradually that 

 they take form and function in the individual ; and the 

 same must be true of their racial evolution. There is 

 contractility before there are definite contractile organs 

 and muscles ; there is diffuse sensitiveness before there are 

 defined nerves and sense-organs. The progress of struc- 

 ture, alike in the individual and in the race, is from 

 simplicity to complexity, as the progress of function is 

 from homogeneous diffuseness to heterogeneous specialisa- 

 tion. The two great kinds of progress may be illustrated 

 by contrasting a sea-anemone and a bird. The higher 

 animal has more numerous parts or organs, the division 

 of labour within its body has brought about more differ- 

 entiation of structure, but it is also a more perfect unity, 

 its parts are more thoroughly knit together and har- 

 monised. There is progress in integration as well as in 

 differentiation. 



" The shoulder-girdle of the skate," W. K. Parker says, " may be 

 compared to a clay model in its first stages, or to the heavy oaken 

 furniture of our forefathers that stood ponderous and fixed by its 

 own massy weight. As we ascend the vertebrate scale, the mass 

 becomes more elegant, more subdivided, and more metamorphosed, 

 until, in the bird class and among mammals, these parts form the 

 framework of limbs than which nothing can be imagined more agile 

 or more apt. So also as regards the sternum ; at first a mere out- 

 crop of the feebly developed costal arches in the amphibia, it becomes 

 the keystone of perfect arches in the true reptiles, then the fulcrum 

 of exquisitely constructed organs of flight in the bird ; and lastly, 

 forms the mobile front wall of the heaving chest of the highest 

 vertebrate." 



5. Classification of Organs. We may arrange organs 

 according to their work, some, such as limbs and weapons, 

 being busied with the external relations of the organism ; 

 others, such as heart and liver, being concerned with 



