56 COMPARATIVE ANATOMY. 



developmental stages in the individual are examples of this kind of 

 reduction (Larval organs). (Cf. § 5.) 



The other kind of reduction affects organs which belong to the 

 developed organism or its rudiments. It may affect the fully 

 formed and completely functional organ as well as one just laid 

 down, and in the primary state of differentiation. The process of 

 reduction is seen therefore in various degrees of intensity. The 

 process is often difficult to perceive amid the various other processes 

 of differentiation which are affecting the rest of the organism when 

 the organ affected is only just making its appearance : the further, 

 however, differentiation has gone, the more striking must the 

 process be. 



The reduction of an organ is necessarily connected with its 

 function, a change in which must be regarded as the cause of the 

 reduction. Loss of function produces retrograde changes in an 

 organ, but of course neither process is a sudden one. 



Although reduction is, on the whole, the cause of the simplifica- 

 tion of an organ, and therefore of the organism also, it is not a 

 phenomenon which makes the organism absolutely lower in degree. 

 Reduction may rather lead to a higher differentiation, as it does when 

 larval organs are removed ; it may give rise to higher forms even in 

 whole series of organisms derived from one another, by facilitating 

 the higher development of those not affected by it. In this case 

 again reduction precedes differentiation. This is strikingly seen in 

 the numerical relations of parts, which become individually more 

 perfect as they diminish in number. 



As reduction is a gradual process, the organs which are affected 

 by it may be met with in various stages. These rudimentary 

 organs are most significant indications of genetic relations, while 

 they at the same time show us how an organ which has lost its 

 primitive function, and which may even have no intelligible signifi- 

 cation as regards the purposes of the organism, may persist for 

 a very long time before it completely disappears. (Cf. supra, § 6.) 



Reduction may affect every organic system and be observable in 

 every part of it. It is expressed in the form as well as in the size 

 and number of the parts, and even in their histological characters. 

 Its conditions are to be sought for, first of all, in the relations 

 which alter the organism. According to the number of organs 

 affected, reduction is more or less manifest in the organism as a 

 whole. 



Inasmuch as comparison everywhere reveals to us evidence of 

 either progressive or of retrogressive change, we may regard the 

 organism as a thing caught in the act of moving, as arrested in 

 the midst of a career through the most diverse ranges of form. 

 The changes of the various organs, and the phenomena which are 

 observed in the elementary structure in the cell, are what make up 

 this movement. 



