10 SYNOPTIC COLLECTION. 



further development was probably effected by a process 

 of specialization. 



This process of specialization may have followed along 

 many lines of development in the successive generations 

 of a species. 



Thus, for example, not only the locomotive organs but 

 also the mouth parts, sense organs, and skeletal structures 

 may have become differentiated, while corresponding inter- 

 nal changes may have taken place, the resulting species 

 possessing manifold organs and functions. Such animals 

 are broadly differentiated species, becoming so by the law 

 of specialization by addition. Secondly, the process of 

 specialization may have followed along a few lines, result- 

 ing in the excessive development of a few organs and the 

 partial or complete loss of others. These animals are 

 specialists by the law of specialization by reduction acting 

 under favorable conditions. Thirdly, the same process 

 under the thwarting influence of an extremely unfavorable 

 environment may have tended, not towards many sided 

 development and the production of a broadly differentiated 

 species ; not towards the excessive development of a few 

 organs with the loss of others and the consequent pro- 

 duction of a more specialized species ; but rather towards 

 a gradual diminution in the size of the organisms, a loss 

 of many organs and functions, and finally towards the 

 gradual extinction of the species or group. Such a spe- 

 cies we shall describe as reduced, since it is truly an 

 extreme product of the law of specialization by reduction. 

 In any one of these three cases mentioned above we have 

 originally primitive forms giving rise to secondary forms, 

 and in a collection based upon natural relationships the 

 former should always precede the latter. We have 

 adhered strictly to this principle in the Synoptic Col- 

 lection of this Museum, so far as the present state of our 

 knowledge would permit. 



In the groups of the Metazoa or animals that follow 

 after the Protozoa, and Mesozoa, the development of the 



