but how much more simple and sublime [a] power

[Tenos]

After Charles Darwin had returned home from the journey of the Beagle, he was reflecting on species and creation, recording his thoughts in his note book, like the following observation:

Astronomers might formerly have said that God foreordered each planet to move in its particular destiny. In the same manner God orders each animal created with certain
forms in certain countries, but how much more simple and sublime [a] power—let attraction act according to certain law, such are inevitable consequences—let animals be created, then by the fixed laws of generation, such will be their successors.

I am perplexed by the structure of the part in bold; I guess it might be: but what a very simple and sublime power is it, that let gravity inflict its inevitable consequences according to a certain law, and create animals, which will produce such successors, according to fixed laws of generation.

 

[reno33]

Highly Simplified Summary:

God may have created animals (along with other living things) but it's much more simple (and easier to believe) that living things came about by fixed laws of generation (evolution) and natural laws of selection.

came about by = came into existence through the process of........

 

[kentix]

He's comparing the intellectual development of a different scientific field, astronomy/physics, with biology. He is saying many years earlier, people believed that the gods determined the paths of each planet individually. Then, many years later, the concept of gravity was discovered and understood and scientists realized that there was a very simple explanation for the way every planet moved. Each planet didn't need its own explanation. The laws of gravity explained all of them.

Darwin is saying in his age people still believed that each animal was a separate, individual creation of God and each lived according to its own rules. He is saying that, just like planets were easier to understand when a few simple rules explained them, he thinks the same is true of animals. His theories of natural selection would explain them and be simpler, like gravity explains the movement of planets.