14 Evolution and Distribution of Fishes 



through subsequent periods up to the present have been 

 preserved. In exactly similar, though in greatly more com- 

 plex manner, definite simpler organic constituents have 

 united into highly complex unions that remain morpholog- 

 ically identical with each other, so long as like environal 

 agencies operate on them and cause like response by the 

 organism. 



In the case of fishes and of the more primitive types 

 that lead up to them, long eons of slow evolutionary action 

 and reaction had to elapse, after the simplest organisms 

 were evolved, before sufficiently complex bodies were built 

 up that could act as the complex progenitors of nemerteans 

 and later of fishes. But at each step in the process definite 

 organisms, characterized by definite structural details, be- 

 came equilibrated to an average environment and thus con- 

 stituted a species-type that reproduced its kind. Thus 

 myriads of individuals might show through long ages a 

 heredity or similarity of detail. 



Alike for nemerteans, for primitive and even degraded 

 fishes like Amphioxus, for cyclostomes, for dipnoans, and 

 for some ganoids, a surprising similarity in hereditary 

 details has evidently persisted through long ages, so that 

 individuals belonging to each of these groups all retain 

 at the present day close resemblances of hereditary struc- 

 ture, even though scattered widely apart over the world. 

 Therefore, though no traces of the first three groups named 

 are surely met with in the fossil state owing to the soft 

 nature of their body-substance, their structure, embryology 

 and distribution clearly proclaim them to be very ancient 

 forms that still exhibit a surprising constancy of heredity 

 for each group. They thus become of high importance, 

 as indicating the pathway of evolution pursued by related 

 forms that have been entirely blotted out, owing to adverse 

 and destructive environal surroundings. 



II. ENVIRONMENT. 



But some of the above individuals, by environal change, 

 might be so altered in their complex organic constituents, 

 that new details of bodily organization would appear. 

 Thus freshwater fishes that live in and are exposed to 



