While "cycling on" could be used in a number of contexts to mean "to continue doing" or "to continue through time," the specific meaning here is given by the next clause:
...cycling on according to the fixed law of gravity...
In this instance, Darwin is poetically (and verbosely) referring to the Earth's orbit. The entire phrase could be replaced as follows (replacement in bold):
...and that whilst Earth has orbited, from...
Even more simply, he's referring to the passage of time (the time spent orbiting the sun).
"Being" is used in the passive present continuous tense
"Are being" is the passive present continuous tense, which means it identifies someone or something having started something in the past and continuing that something in the present, perhaps ending in the future. Thus, the last bit could be written:
...endless forms most beautiful and wonderful have evolved and continue evolving.
It is worth noting that Darwin defined evolution as a reaction to the surrounding environment. In the case of this sentence, he is anthropomorphising "the environment." Thus, those endless forms are being evolved by the influence or effect of the environment they live in.
As for rephrasing the whole sentence. It's actually easier to just explain each component.
There is a grandeur in this view of life Thinking about or considering the perspective Darwin presented in his treatise suggests something amazing about the nature of life.
with its several powers His presentation of evolution and adaptation suggest that life has powerful abilities not considered in other views of life.
having originally breathed into a few forms or one This is a bit of a jab at Christianity, whose views on creationism were opposed to Darwin's views of evolution and adaptation. From the Christian perspective, life was "breathed" into Man by God. Darwin's treatise suggests an evolution from a very few simple lifeforms into our complex world of life. He borrowes "breathed" both to make the statement contextually understandable and to poke his detractors a bit.
and that whilst Earth has gone cycling on according to the fixed law of gravity explained above.
from so simple a beginning Darwin proposes that we evolved from one-celled life, which is the most simple of beginnings.
endless forms most beautiful and wonderful "Endless forms" refers to the complexity of life both today and throughout time, which are, indeed, beautiful and wonderful.
have been, and are being, evolved. Explained above.
