DEVELOPMENT AND DEGENEEATION. 55 



other directions of development. A limb which is a gill too, that is 

 which has both locomotive and respiratory functions, is of a lower 

 grade than an arrangement resulting from a division of the two 

 functions, in which a part separated off from the appendage repre- 

 sents a gill, and the rest an organ of locomotion. When the functions 

 are united, locomotion is necessary for respiration, but when they 

 are separated, they are independent of one another, and respiration 

 is effected without the aid of locomotion, by the development of 

 special organs for changing the water, these organs so far taking 

 the place of locomotion. In both organs the independence which is 

 necessary for their further development in one direction is gained. 



The organs of the body are not always developed to the same 

 extent. One or another often remains in a lower condition, and so 

 retains its more lowly character in an otherwise highly differentiated 

 organism. It is not therefore wise to draw any conclusions as to 

 the extent of the differentiation of single organs from that of the 

 organism itself; it is better to judge organs by comparing them 

 with equivalent organs in other organisms. 



The real factor in the development of an organ by differentiation 

 must be sought for in the increased or modified function of the organ 

 in the struggle for existence, that is in its adaptations to the ex- 

 ternal conditions of life. It is hence that transmission acquires its 

 importance, since it not only causes a perpetuation of inherited 

 characters, but is enabled to effect an elevation in those characters. 

 Degeneration or reduction is another constant phenomenon 

 which is dependent on differentiation, inasmuch as it presupposes it. 

 Its result is, in itself, the exact opposite to that of differentiation. For 

 while differentiation is the cause of complications, reduction is the 

 cause of simplifications of the organism, and is therefore the cause 

 of organs or of organisms passing to a relatively lower stage. With 

 regard, however, to the general organism, and its relation to other 

 organisms, it produces the same effect as differentiation, for it leads 

 to variety in form. 



Reduction, like differentiation, varies in degree; it may affect 

 separate portions of the body, or groups of organs, or finally the 

 whole of the body. It is different, again, according as it affects 

 the individual, the species, or the genus. In one case it may be 

 seen to be a definite process, in another a condition, which can only 

 be assigned its place as one of the several stages of such a process 

 by the aid of a comparative series of allied forms. It may affect 

 organs in two different ways. The affected organ may be in- 

 dependent of the general arrangements which obtain in the de- 

 veloped organism to which it belongs, and then reduction has but 

 a transitory or provisional signification. Reductions of this kind 

 during the course of the development produce simplifications, but as 

 the differentiation which is going on in other parts may be producing 

 new and higher organs, this reduction does not hold the organism 

 back, but is a cause rather of the progress of differentiation in 

 another direction. The reduction of parts which belong to certain 



