EVOLUTION, VARIATION AND HEREDITY 449 



modified, blurred and incomplete. For while there is a circulatory 

 system in the chick similar to that in a water-dwelling fish, yet the 

 embryo chick is not a fish, and could not utilise this arrangement for 

 aquatic respiration. The history is often enormously modified to 

 suit the actual requirements of the young animaL and so on. 



Striking applications of this law are to be met with in certain 

 groups of the animal kingdom. Thus in the Cirripedia, a group 

 containing the ship barnacles and the rock barnacles, and in the 

 parasitic Copepoda, a group parasitic on fish, crabs, etc., the adult 

 animal gives practically no indication of its relationships. Yet in 

 both cases its life history shows that it is really a much modified 

 crustacean, i.e. an animal allied to shrimps, etc. The sea squirts 

 or Ascidians, almost shapeless masses of leathery jelly on the rocks, 

 were also classified satisfactorily when their life history had been 

 elucidated. Thus this law has aided in placing various groups in 

 their proper position in the animal series, because their developmental 

 history has revealed their relationship to other forms. 



When there is doubt as to the exact place that any species holds 

 in a genus, use can often be made of the fact that generally the 

 embryos of closely related species resemble one another more nearly 

 than those of less closely related species or than the adults themselves 

 do. Similarly we find that embryos of closely related groups tend 

 to resemble one another more closely than the adult members of fhe 

 group. 



It will be seen then that all these phenomena point to the 

 conclusion that the present-day forms have been derived from 

 less specialised pre-existing forms, and that from one such ancient 

 generalised type a number of species have arisen. There is no logical 

 reason to stop at the species, but larger and larger groups with a com- 

 mon structural basis have come from a more remote form, and so on. 



Geographical Evidence. 



On studying the geographical distribution of living beings 

 we find that closely allied animals and plants are usually to be 

 found living in neighbouring districts. Conversely, large tracts of 

 land usually contain a number of allied species fairly widely 

 scattered over it. When, however, a sea or a mountain range or a 

 desert or some such " barrier " that has been in position for a long 

 time, geologically speaking, intervenes, the animals and plants on 

 the two sides of it are in general not closely related. This is because 

 the closely allied species have had a common origin and have 

 spread and spread until further expansion has been hindered by 

 barriers. If closely allied species were not circumscribed in this 

 way, and had not originated in the same place, there is no reason 



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