18 NATURAL SELECTION I 
There must be a cause for them; they must be the necessary 
results of some great natural law. Now, if, as it has been 
endeavoured to be shown, the great law which has regulated 
the peopling of the earth with animal and vegetable life is, 
that every change shall be gradual; that no new creature 
shall be formed widely differing from anything before exist- 
ing; that in this, as in everything else in nature, there shall 
be gradation and harmony,—then these rudimentary organs 
are necessary, and are an essential part of the system of 
nature. Ere the higher Vertebrata were formed, for instance, 
many steps were required, and many organs had to undergo 
modifications from the rudimental condition in which only 
they had as yet existed. We still see remaining an antitypal 
sketch of a wing adapted for flight in the scaly flapper of the 
penguin, and limbs first concealed beneath the skin, and then 
weakly protruding from it, were the necessary gradations 
before others should be formed fully adapted for locomotion.1 
Many more of these modifications should we behold, and more 
complete series of them, had we a view of all the forms which 
have ceased to live. The great gaps that exist between fishes, 
reptiles, birds, and mammals would then, no doubt, be 
softened down by intermediate groups, and the whole organic 
world would be seen to be an unbroken and harmonious 
system. 
Conclusion 
It has now been shown, though most briefly and imper- 
fectly, how the law that “ Every species has come into existence 
coincident both in time and space with a pre-existing closely allied 
species,” connects together and renders intelligible a vast 
number of independent and hitherto unexplained facts. The 
natural system of arrangement of organic beings, their geo- 
graphical distribution, their geological sequence, the pheno- 
mena of representative and substituted groups in all their 
modifications, and the most singular peculiarities of anatomical 
structure, are all explained and illustrated by it, in perfect 
accordance with the vast mass of facts which the researches of 
modern naturalists have brought together, and, it is believed, 
1 The theory of Natural Selection has now taught us that these are not the 
steps by which limbs have been formed ; and that most rudimentary organs 
have been produced by abortion, owing to disuse, as explained by Mr. Darwin. 
