Darwinism is an effort to explain the diversity of life by natural causes.
Darwinism is based on Charles Lyell's theories of geology that everything happens by slow gradual changes (uniformtarism )
Darwin extrapolated this principle to living things. He believed that all life started with a "simple cell" ( not realizing how complex the simplest cell actually is) Then by slow gradual changes the cell changed into all living things that we see today.
Darwin was influenced also by the French and English Enlightenment that sought only natural causes as explanations for everything. So Darwinism is an effort to explain the origin and diversity of life by only natural causes.
Darwin observed that by artificial selection animals and plants could be changed dramatically in their form and appearance. In his book the Origin of the Species Darwin spends a great deal of time detailing the changes that artificial selections could achieve in the domestic pigeon. Darwinism extrapolates these changes to being able to happen in nature by Natural Selection.
Darwin observed that living organisms produced far more offspring than could be supported. All the offspring could not survive so nature would "select" the offspring based suited to survive. The organism that had changed to be best adapted to the environment would survive and pass on these changes to their offspring. hence the origin of a "new species".
Darwinism postulates infinite possible variations of all organisms. Giving the immense variations possible in the mixing and reshuffling of genes this was not an unreasonable assumption in Darwin's time.
However with the advent of the theories and discoveries of genetics it was soon realized that there is not an infinite variation of existing genes. There could be changes within a species by natural selection but not the creation of a new species.
The answer postulated by Neo Darwinism was mutations. Mutations were proposed as the source of new species. Over 100 years of experiments with fruit flies has not produces any new species only harmful variations of fruit flies. Still Neo Darwinism must rely on faith that given enough time mutation could result in new species.
The discovery of the information code of DNA has created new problems for Darwinism. How information can be spontaneous created by natural causes seems to go against the laws of information theory which has found that laws of entropy apply to information transfer.