"It was always understood that plants and animals, though completely contrasted in their higher representatives, approached each other very closely in their lower and simpler forms. But they were believed not to blend"

Asa Gray's quote highlights a fascinating point of view on biological classification and the evolutionary continuum in between plants and animals. This declaration reflects the clinical understanding of the time relating to the relationship between these two basic classifications of life.

Gray begins by acknowledging that, at higher levels of complexity, plants and animals appear significantly different, showcasing unique qualities and having distinct roles within their communities. Greater representatives of plants, such as flowering plants and big trees, are rooted in location, utilizing sunshine to produce energy through photosynthesis and often having complex reproductive techniques involving pollen and seeds. Likewise, animals like mammals, birds, and fish, display movement, complex behavior, and unique biological systems adjusted for consuming raw material for energy.

However, Gray mentions that these distinctions become less pronounced as we look at simpler and more primitive types. At these lower levels, the fundamental attributes of life blur, and organisms might not fit neatly into one classification or the other. For example, some lower plants like algae, as well as simple animals like sponges, display characteristics that do not strictly abide by the defining characteristics of either kingdom, showing their shared evolutionary origins.

Gray concludes by mentioning the prevailing belief of his time: while these forms approach each other carefully, they were comprehended not to mix into one another. This speaks with the idea that, despite their resemblances, there was a dividing line that avoided them from combining into a constant spectrum. During Gray's period, this line was typically perceived as more categorical than we understand it today, with a more stiff separation between plant and animal kingdoms.

This quote hence catches a moment in clinical idea where the boundaries between plant and animal life were actively explored and questioned, setting the phase for further study into the evolutionary processes that eventually connect all living organisms. Today, with improvements in genes and molecular biology, we comprehend that the tree of life is more of an intricate web, with numerous instances of overlap, horizontal gene transfer, and a clearer understanding of ancestral relationships that challenge these earlier conceptions.