All you have to do is see Wiki's entry on speciation and you will see that all the evidence is for adaptive changes within a species. You will never see one species evolving into another completely unrelated species. The fish were still fish...the flies were still flies.
What did Darwin observe on the Galapagos Islands? He saw species different to the ones on the mainland....but the birds were still birds...the tortoises were still tortoises....the iguanas were still iguanas. He saw adaptation, not organic evolution.
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Darwin's dilemma: Why do species exist?
In
On the Origin of Species (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the absence or rarity of transitional varieties in habitat space.
Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species."
https://en.wikipedia.org/wiki/Speciation
Darwin asked " why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?"
So where are the intermediate forms that should exist if evolution is true? Why is the planet not riddled with their remains?