Sentence:  Indeed we have always thought that Mr. Darwin has unnecessarily hampered himself by adhering so strictly to his favourite “Natura non facit saltum.” We greatly suspect that nature does make considerable jumps in the way of variation now and then, and that these <WSD>saltations</WSD> give rise to some of the gaps which appear to exist in the series of known forms. 
Meaning of the word marked with <WSD> tags:  A sudden change from one generation to the next; a mutation.