Darwin often used the phrase "_[blank]_" to suggest that species must have descended from preexisting species and that changes in the species took place over long periods of time  on the origin of species  descent with modification  descent with differential reproduction  survival of the fittest

Descent with modification

Explanation:

Darwin proposed the phrase "descent with modification" to explain that evolution of new species occurs when existing species accumulate genetic variation over the period of time.  

Accordingly, the process of accumulation of genetic variations to get better adapted to the surroundings is followed by natural selection of individuals with beneficial adaptations that impact survival advantage to them. These individuals transmit the adaptations to their progeny.

Gradually, the process leads to the evolution of new species which differ from parental species in having the individuals carrying the beneficial adaptations.  

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explanation:

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.