24 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



Zoology is the science which has animals for its subject, and which 

 seeks to examine the phenomena of their structure and life, as well 

 as their relations to one another and to the outer world. 



CHAPTER III. 



THE ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



In the foregoing comparison of animals and plants for the 

 establishment of a correct idea of the meaning of the word " animal," 

 the great variety and the numerous grades of animal structure have 

 been hinted at. Just as the complex organism is built up from the 

 ovum by a process of gradual differentiation, and often during its 

 free life passes through conditions which lead in ascending series 

 to an ever higher development of the parts and to a more complete 

 performance of functions; so, if the animal kingdom be examined as 

 a whole, there is apparent a similar law of gradually progressing 

 development, of an ascent from the simple to the complex, manifest 

 both in the form of the body and in the cornpositicn of its parts as 

 well as in the completeness of the phenomena of life. 



It is true that the grades of animal structure do not, like those of 

 the developing individual, follow the one upon the other in a single 

 continuous series ; and the parallel between the developmental 

 gradation of types in the animal kingdom as a whole and the suc- 

 cessive conditions of an individual animal breaks down in so far as 

 we distinguish in the former, as opposed to the latter, a number of 

 types of animal structure often overlapping, but still, in their higher 

 development, essentially different from each other. These we regard 

 as the highest divisions of the system. 



INDIVIDUAL ORGAN STOCK. 



The animal organism, when viewed from a physiological and mor- 

 phological stand-point, presents itself as an independent and indivisible 

 unit, as a " complete individual." Amputated limbs or excised parts 

 of the body do not develop into new animals ; in fact we cannot 

 usually remove a single piece of the body without thereby endanger- 

 ing the life of the organism, for it is only as a complex of all its 

 parts that the body can retain its full vital energy. With reference 

 to the property of the indivisibility of the individual, we understand 



