contraction hypothesis

例句：

1. 1. The theory of evolution by natural selection is a well-established scientific hypothesis.
   自然选择的进化理论是一个广为接受的科学假设。
2. 2. This model assumes that the population size will decrease over time due to natural causes.
   这个模型假设由于自然原因，人口数量将随着时间的推移而减少。
3. 3.The researchers proposed a new hypothesis about how diseases spread through the environment.
   研究人员提出了一种新的关于疾病如何通过环境传播的假设。