CHAP. XI The Elements of Structure 175 



from bilaterally symmetrical worms, lobsters, fishes, and 

 most other animals. Then there is the difference between 

 unsegmented animals which are all one piece (like the 

 lower worms and the molluscs), ani those whose bodies 

 consist, as in earthworm and crayfish, of a series of more or 

 less similar rings or segments, due to conditions of growth 

 of which we know almost nothing. 



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. Gradually, alike in the individual 

 and in the race, do they take form and function. There is 

 contractility before there are definite contractile organs or 

 muscles ; there is diffuse sensitiveness before there are 

 defined nerves or sense-organs. The progress of structure, 

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

 to complexity, as the progress of function is from homo- 

 geneous diffuseness to heterogeneous specialisation. 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 differentiation of structure, 

 but it is also a more perfect unity, its parts are more 

 thoroughly knit together and harmonised. There is pro- 

 gress 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 frame- 

 work of limbs than which nothing can be imagined more agile or 

 more apt. So also as regards the sternum ; at first a mere outcrop 

 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. " 



Of the order in which organs appear or have appeared 

 we can say little. The simplest sponges and polypes are 



