Variation and Fixity in Nature

In nature we observe both diversity and discontinuity among all plants and animals. Living things cannot be arranged in a continuous, unbroken series from simple to complex, nor can one variant be traced through a continuous series to a markedly different variant. 

What do the phenomena of diversity and discontinuity say to us as to the origin and meaning of living things? How do the observable facts fit in with the various theories of origins? What are the strengths and weaknesses of each?