To be viewed from the perspective of epistemology and/or the philosophy of science.
Preamble:
Darwin, like scientists of his day, often spoke of “Laws” that inevitably cycle forth the results implicit in their form.
Question:
In what ways did Darwin delve into the question of why there is life as opposed to a strictly abiotic world?
Did he try to define such a Law of biology that would answer the “why”?
One has to keep apart different layers: a) abiogenesis (the emergence of life) vs. evolution (the development of existing life over generations) and b) the incompatibility of biblical accounts of the origin of species with evolution vs. the incompatibility of the belief in God being the creator of life with evolution.
I will first answer the title question and then address the second layer.
Charles Darwin himself did never explicitly write that it is the case that life developed gradually by natural processes, but this was due to the impossibility to scientifically prove this possibility at the time. It was his personal conviction, though: he did not believe in teleological or theological explanations of life.
This point is made and shown as valid in the paper: Peretó, J., Bada, J. L., & Lazcano, A. (2009). Charles Darwin and the Origin of Life. Origins of Life and Evolution of the Biosphere, 39(5), 395–406. http://doi.org/10.1007/s11084-009-9172-7
The abstract reads:
When Charles Darwin published The Origin of Species 150 years ago he consciously avoided discussing the origin of life. However, analysis of some other texts written by Darwin, and of the correspondence he exchanged with friends and colleagues demonstrates that he took for granted the possibility of a natural emergence of the first life forms. As shown by notes from the pages he excised from his private notebooks, as early as 1837 Darwin was convinced that “the intimate relation of Life with laws of chemical combination, & the universality of latter render spontaneous generation not improbable”. Like many of his contemporaries, Darwin rejected the idea that putrefaction of preexisting organic compounds could lead to the appearance of organisms. Although he favored the possibility that life could appear by natural processes from simple inorganic compounds, his reluctance to discuss the issue resulted from his recognition that at the time it was impossible to undertake the experimental study of the emergence of life.
They use letters and notes. One of the last letters is quoted in full towards the end of the paper and reaffirms the point:
He was to maintain the same attitude for many years to come, as shown by the letter mailed on March 28, 1882, near the end of his life, to George Charles Wallich (de Beer 1959). In it Darwin wrote that,
«My dear Sir,
You expressed quite correctly my views where you say that I had intentionally left the question of the Origin of Life uncanvassed as being altogether ultra vires in the present state of our knowledge, & that I dealt only with the manner of succession. I have met with no evidence that seems in the least trustworthy, in favour of the so-called Spontaneous generation. I believe that I have somewhere said (but cannot find the passage) that the principle of continuity renders it probable that the principle of life will hereafter be shown to be a part, or consequence of some general law; but this is only conjecture and not science. I know nothing about the Protista, and shall be very glad to read your Lecture when it is published, if you will be so kind as to send me a copy.
I remain, my dear Sir,
Yours very faithfully
Charles Darwin»
Thus, Darwin was convinced, but not able to scientifically demonstrate, that life emerged following laws of nature, i.e. "naturally". Consequently, he withheld this conviction in his scientific publications. As such, Darwin was a true scientist.
Darwin himself seemed to be against the idea that God did create life. At least, he saw no conclusive reason to think that he did. The authors of the paper referred to earlier write:
In a letter he sent in February 28, 1882 to D. Mackintosh (Letter 13711, Cambridge University Library, DAR.146:335), he included an indirect reference to Wöhler’s synthesis of urea and added that
«[...] If it is ever found that life can originate on this world, the vital phenomena will come under some general law of nature. Whether the existence of a conscious God can be proved from the existence of the so called laws of nature (i. e. fixed sequence of events) is a perplexing subject, on which I have often thought, but cannot see my way clearly...».
But this does not mean that belief in God is incompatible with evolutionary theory in principle, although standing at odds with the descriptions given in the Bible. As written in Stewart-Williams, S. (2010). Darwin, God and the Meaning of Life: How Evolutionary Theory Undermines Everything You Thought You Knew. Cambridge: Cambridge University Press (a recommended read for all those interested in this problem):
From the moment Darwin put forward his theory, it was obvious to his contemporaries that, if he was right, the biblical account of the origin of life must be wrong. In principle, people might have concluded that the Bible was wrong about the details of how God created life, but not about the fact that God existed in the first place. (p.44)
Note: Philip Klöcking answers the title question, this answers the question in the body.
The original work of Darwin was named "On the origin of species by means of natural selection", and not "On the origin of life".
Evolution takes place after Abiogenesis.
So asking someone trusts that evolution by natural selection is an accurate model for speciation what they think about the origin of life, is kind of like asking an electrician what they think about a power station. The subject matters are related, yes... without electricity there would not be any need for the electrician to do any work. Similarly, without an origin of life there could be no evolutionary process.
But they are still two different fields of interest. At the power station electricity is created, but out at the substations and in the buildings of end-users, is where the electricity is routed, distributed and used.
The godly use two arguments (among many others) to try to prove their case for a deity:
The theory of evolution refutes only the latter statement. It does not validate or invalidate the first statement. Presently there are hypotheses about the origin of life, but no established scientific theory about the origin of life.
These are two different arguments and they are validated/invalidated separately. You can have a creator that made life, but then did not meddle with evolution. You can have a designer that took preexisting life and started meddling with it. These are two different matters that do not necessarily reinforce each other. .
People that regard the theory of evolution as an accurate model of speciation are usually scientifically minded. Scientifically minded people will — when asked about a matter, and there is no theory on the subject — commonly answer: "I do not know".
And such is the case here.
You say:
why life was formed
Science never asks "why". It asks "how". "How does gravity work?". "How does evolution work?". "How did/do the stars form?". "How did life originate?".
Asking "why" implies you are looking for a purpose. Science does not look for purpose. It looks for an understanding of how reality works.
(This is not a philosophy question. It has no philosophical content of its own. It is straightforward question about the history and current state of biological science.)
Darwin's noted solution here, spontaneous generation, is basically wrong. It is incredibly unlikely that life should spring, fully functional, from non-biological matter. There is intermediate biological matter like viruses, which follow the laws of evolution, but are not yet life forms. Without modern technology, he could not have known that.
So the most common evolutionary answer to this question avoids Darwin and starts from DNA: Life started because given enough time self-replicating molecules were unavoidable. Once you have self-replicating molecules, they create a competition for the resources out of which they are made. Life forms arise naturally as the simplest solutions to the problems of how to collect, store and re-use energy for competition.
Check out Richard Dawkins and the theory of the selfish gene.
There is a point in human understanding where science and philosophy (people also interchangeably state 'religion') meet. That meeting point is the key to answering the question on origin of life. The 'why' and 'how' in this case can both be answered if we combine these two areas of thought. Charles Darwin definitely was looking at an empirical way of observations - scientific experiments with technology and understanding developed by that time - to answer the question; hence, he was not able to provide an explanation on the origin of life ... perfectly understandable. For the answer itself, that would be another question which can be answered :-)
