This is a very interesting set of questions. I share the same questions with Arnab. I can only surmise that during Darwin's time, the use of the scientific method was not as detailed as the research Methodology we employ today. The writings of Darwin, however are published and open for critic.

Darwin did not employ probability sampling. He did not even have a sampling frame. He did not calculate his sample size with margin of error or confidence level. He did not apply random selection. Effect size, I guess, was not a popular expression among scientists then,

This is a set of questions that could with equal unfairness be applied to Linnaeus, Mendel or any other scientist of that time (biologist, physicist, whatever). It is IMHO Whiggish to take current standards of experimental research, apply them to earlier scientists, find that they did not conform to those standards and thereby tacitly imply that they must have been wrong.

How large is the sample of an astronomer observing a supernova? n=1

Prediction in biology works like this: 

1. Heritable traits have been observed -> factors of inheritance are 'predicted', a material medium is postulated.

2. Chromosomes are being made out as that material medium for heritable factors -> a structure that allows replication is being 'predicted'.

And so on, until you see that Watson and Crick did indeed predict the structure of the DNA based on tinkered paperboard models. Though that is nowadays always called a 'discovery' rather than a 'prediction'.

P.S.: Prediction in biology is not clairvoyance about what will evolve.

all i want to say is that mendel's theory make accurate prediction which gives birth to the science of genetics. how many accurate prediction darwin's theory has made to date?

if it fails to make successful prediction then why should we hold this theory as a scientific fact? what purpose does it serve then?

P.S: please don't say that the credibility of theory does not depend on prediction.

some more tests of scientific theory apart from prediction:

1. it must be supported by many evidence rather than one evidence.

2. it is consistent with pre existing theories.

3.It can be adapted and modified to account for new evidence as it is discovered.

did theory of evolution meet any of the above mentioned criteria?

Darwin primarily relied on abductive logical reasoning in coming up with the theory of evolution.  He also made a number of mistakes, such as with his suggestions on the mechanism of inheritance.

if he made a number of mistake, then his theory is not very reliable. isn't it?

Subsequent scientific observations have essentially confirmed the basic premise of Darwin's theory of evolution by natural selection.  Darwin wasn't a prophet, he was a naturalist.  Even though Darwin's ideas of how inheritance might occur were essentially wrong, Gregor Mendel was able to show inheritance from parents to offspring, which supports evolutionary theory. 

Evidences were not that important for Darwin. It was found an angiosperm plant which was believed to be before from a period before the time the other plants in the line of evolution. Instead of saying his theory was wrong, he said that such a finding was abominable.

Our article published in peer-reviewed Journal "Communicative & Integrative Biology". A few major points discussed in the paper:

(1) Brain is not the source of consciousness.

(2) Consciousness is ubiquitous in all living organisms, starting from bacteria to human beings.

(3) The individual cells in the multicellular organisms are also individually cognitive entities.

(4) Proposals like “artificial life”, “artificial intelligence”, “sentient machines” and so on are only fairytales because no designer can produce an artifact with the properties like internal teleology (Naturzweck) and formative force (bildende Kraft).

(5) The material origin of life and objective evolution are only misconceptions that biologists must overcome.

http://www.tandfonline.com/doi/full/10.1080/19420889.2015.1085138

There should be a profound revision about natural selection. Darwin had a black and white photogram back then. Right now, we have an entire colorful video. We have got sequences, technology, and greater precision. Nowadays, it is our responsibility to jump ahead. Darwin did his Job.

The human DNA existed before the earth scenario and evolutionary chaos appears also as an important mechanism. The following article not only for mankind, but life in general, exhibits how under certain general and similar limits, there are infinite possible variations through a selection process that it is no Darwinian natural selection or evolution. Many different evolutionary trajectories from innumerable combinations of basic elements occur from the micro to the macroscopic scale.

Article Evolution through the stochastic dyadic Cantor Set: the uniq...