314 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q3. 



which there are either supernumerary or confused parts; because, if we cannot 

 distinctly perceive the use, or necessity of parts, in their natural state, we are 

 not likely to advance in information by the examination of those varieties of 

 structure, where difficulties are only multiplied by the greater complication, or 

 aggravated by the confusion of parts. The only useful inference in natural his- 

 tory, which can be drawn from monsters of the last kind is, that nature can 

 deviate from the usual arrangement of parts, without any material inconvenierjce; 

 and therefore, that the existence of parts so as to be capable of being applied to 

 the purpose for which they are intended, in the perfect state of the system, 

 rather than any precise order of them, is required for carrying on the functions 

 of an animal body. 



Monsters however where considerable parts are wanting, seem peculiarly likely 

 to assist in the prosecution of physiological researches. If we were never to see 

 an animal except in its perfect state, we could form no just idea of the compa- 

 rative necessity of the different parts. So also, if we were to attend alone to 

 the complete structure which obtains in the more perfect animals, we might be 

 led falsely to conclude, that the usual connexion of parts, which we find in 

 them, was essential to the structure and composition of animal matter. Of 

 these parts, the brain and nerves, the stomach and digestive organs generally, 

 the heart, and the lungs, would appear to be of such importance in the machine, 

 that one would be induced to imagine that the functions of life could not be 

 carried on without them: but in tracing the works of nature downwards, we 

 shall at length find animals gradually becoming more and more simple in their 

 construction. The brain and nervous system are altogether wanting in some, 

 and there are others which have neither heart nor lungs; yet they continue to 

 exist, and are capable of performing the most important functions of animals. 

 Thus the formation of one animal serves to throw light on the economy of 

 others. This great simplicity of structure is found however chiefly in animals 

 the texture of whose bodies is nearly homogeneous; not consisting, as in more 

 perfect animals, of parts so different from each other, as skin, intestines, &c. 

 are from bone. It might therefore still be supposed, that all the complicated 

 mechanism, found in the more perfect animals, is essential to the construction 

 of such heterogeneous substances as those of which they consist. 



To investigate this matter, we must have recourse to those monsters in which 

 there is a deficiency of parts. There is a very material difference between the 

 nature of the life of the more perfect animals, during their time of foetal ex- 

 istence, and after they are born. In the latter state, the brain and nerves appear 

 to be so essential, that any very considerable defect in them is incompatible with 

 the well-being of the animal; but in the uterine state, considerable deviations 

 from the ordinary arrangement of parts, and such as cannot be endured after 



